12/11/2025 at 11:43:44 AM
One of my biggest points of criticism of Python is its slow cold start time. I especially notice this when I use it as a scripting language for CLIs. The startup time of a simple .py script can easily be in the 100 to 300 ms range, whereas a C, Rust, or Go program with the same functionality can start in under 10 ms. This becomes even more frustrating when piping several scripts together, because the accumulated startup latency adds up quickly.by randomtoast
12/11/2025 at 1:58:51 PM
Yes, that is also my feeling. But comparing an interpreted language with a compiled one is not really fair.Here is my quick benchmark. I refrain from using Python for most scripting/prototyping task but really like Janet [0] - here is a comparison for printing the current time in Unix epoch:
$ hyperfine --shell=none --warmup 2 "python3 -c 'import time;print(time.time())'" "janet -e '(print (os/time))'"
Benchmark 1: python3 -c 'import time;print(time.time())'
Time (mean ± σ): 22.3 ms ± 0.9 ms [User: 12.1 ms, System: 4.2 ms]
Range (min … max): 20.8 ms … 25.6 ms 126 runs
Benchmark 2: janet -e '(print (os/time))'
Time (mean ± σ): 3.9 ms ± 0.2 ms [User: 1.2 ms, System: 0.5 ms]
Range (min … max): 3.6 ms … 5.1 ms 699 runs
Summary
'janet -e '(print (os/time))'' ran
5.75 ± 0.39 times faster than 'python3 -c 'import time;print(time.time())''
by smartmic
12/11/2025 at 5:09:39 PM
Well python is also compiled technically.by curiousgal
12/11/2025 at 12:17:41 PM
> The startup time of a simple .py script can easily be in the 100 to 300 ms rangeI can't say I've ever experienced this. Are you sure it's not related to other things in the script?
I wrote a single file Python script, it's a few thousand lines long. It can process a 10,000 line CSV file and do a lot of calculations to the point where I wrote an entire CLI income / expense tracker with it[0].
The end to end time of the command takes 100ms to process those 10k lines, that's using `time` to measure it. That's on hardware from 2014 using Python 3.13 too. It takes ~550ms to fully process 100k lines as well. I spent zero time optimizing the script but did try to avoid common pitfalls (drastically nested loops, etc.).
by nickjj
12/11/2025 at 1:44:19 PM
> I can't say I've ever experienced this. Are you sure it's not related to other things in the script? I wrote a single file Python script, it's a few thousand lines long.It's because of module imports, primarily and generally. It's worse with many small files than a few large ones (Python 3 adds a little additional overhead because of needing extra system calls and complexity in the import process, to handle `__pycache__` folders. A great way to demonstrate it is to ask pip to do something trivial (like `pip --version`, or `pip install` with no packages specified), or compare the performance of pip installed in a venv to pip used cross-environment (with `--python`). Pip imports literally hundreds of modules at startup, and hundreds more the first time it hits the network.
by zahlman
12/11/2025 at 8:05:22 PM
Makes sense, most of my scripts are standalone zero dependency scripts that import a few things from the standard library.`time pip3 --version` takes 230ms on my machine.
by nickjj
12/11/2025 at 8:17:05 PM
That proves the point, right?`time pip3 --version` takes ~200ms on my machine. `time go help` takes 25, and prints out 30x more lines than pip3 --version.
by maccard
12/11/2025 at 9:33:21 PM
Yep, running time on my tool's --version takes 50ms and funny enough processing 10k CSV lines with ~2k lines of Python code takes 100ms, so 50ms of that is just Python preparing things to run by importing 20 or so standard library modules.by nickjj
12/12/2025 at 2:05:35 AM
> so 50ms of that is just Python preparing things to run by importing 20 or so standard library modules.Probably a decent chunk of that actually is the Python runtime starting up. I don't know what all you `import` that isn't implied at startup, though.
Another chunk might be garbage collection at process exit.
by zahlman
12/11/2025 at 4:34:40 PM
And it's worse if your python libraries might be on network storage - like in a user's homedir in a shared compute environment.by fwip
12/11/2025 at 6:03:01 PM
Exactly this. The time to start python is roughly a function of timeof(stat) * numberof(stat calls) and on a network system that can often be magnitudes larger than a local filesystem.by dekhn
12/11/2025 at 6:37:45 PM
I do wonder, on a local filesystem, how much of the time is statting paths vs. reading the file contents vs. unmarshaling code objects. (Top-level code also runs when a module is imported, but the cost of that is of course highly module-dependent.)by zahlman
12/11/2025 at 7:02:14 PM
Maybe you could take the stat timings, the read timings (both from strace) and somehow instrument Python to output timing for unmarshalling code (or just instrument everything in python).Either way, at least on my system with cached file attributes, python can startup in 10ms, so it's not clear whether you truly need to optimize much more than that (by identifying remaining bits to optimize), versus solving the problem another way (not statting 500 files, most of which don't exist, every time you start up).
by dekhn
12/11/2025 at 1:46:03 PM
Here is a benchmark https://github.com/bdrung/startup-timeThis benchmark is a little bit outdated but the problem remains the same.
Interpreter initialization: Python builds and initializes its entire virtual machine and built-in object structures at startup. Native programs already have their machine code ready and need very little runtime scaffolding.
Dynamic import system: Python’s module import machinery dynamically locates, loads, parses, compiles, and executes modules at runtime. A compiled binary has already linked its dependencies.
Heavy standard library usage: Many Python programs import large parts of the standard library or third-party packages at startup, each of which runs top-level initialization code.
This is especially noticeable if you do not run on an M1 Ultra, but on some slower hardware. From the results on Rasperberry PI 3:
C: 2.19 ms
Go: 4.10 ms
Python3: 197.79 ms
This is about 200ms startup latency for a print("Hello World!") in Python3.
by randomtoast
12/11/2025 at 2:05:32 PM
Interesting. The tests use Python 3.6, which on my system replicates the huge difference shown in startup time using and not using `-S`. From 3.7 onwards, it makes a much smaller percentage change. There's also a noticeable difference the first time; I guess because of Linux caching various things. (That effect is much bigger with Rust executables, such as uv, in my testing.)Anyway, your analysis of causes reads like something AI generated and pasted in. It's awkward in the context of the rest of your post, and 2 of the 3 points are clearly irrelevant to a "hello world" benchmark.
by zahlman
12/11/2025 at 1:58:07 PM
A python file with import requests
A go program with
package main
import (
_ "net/http"
)
func main() {
}
by maccard
12/11/2025 at 2:13:41 PM
This is not an apples-to-apples comparison. Python needs to load and interpret the whole requests module when you run the above program. The golang linker does dead code elimination, so it probably doesn't run anything and doesn't actually do the import when you launch it.by dotdi
12/11/2025 at 3:16:07 PM
Sure it's not an apples to apples comparison - python is interpreted and go is statically compiled. But that doesn't change the fact that in practice running a "simple" python program/script can take longer to startup than go can to run your entire program.by maccard
12/11/2025 at 5:06:37 PM
Still, you are comparing a non-empty program to an empty program.by dotdi
12/11/2025 at 7:43:37 PM
Even if you actually use the network module in Go, just so that the compiler wouldn't strip it away, you would still have a startup latency in Go way below 25 ms from my experience with writing CLI tools.Whereas with Python, even in the latest version, you're already looking at atleast 10x the amount of startup latency in practice.
Note: This is excluding the actual time that is made for the network call, which can of course also add quiete some milliseconds, depending on how far on planet earth your destination is.
by tuhgdetzhh
12/11/2025 at 8:09:45 PM
You're missing the point. The point is that python is slow to start up _because_ it's not the same.Compare:
import requests
print(requests.get("http://localhost:3000").text)
package main
import (
"fmt"
"io"
"net/http"
)
func main() {
resp, _ := http.Get("http://localhost:3000")
defer resp.Body.Close()
body, _ := io.ReadAll(resp.Body)
fmt.Println(string(body))
}
python3: 0.08s user 0.02s system 91% cpu 0.113 total
go 0.00s user 0.01s system 72% cpu 0.015 total
I wrote another that counts the lines in a file, and tested it against https://www.gutenberg.org/cache/epub/2600/pg2600.txt
I get:
python 0.03s user 0.01s system 83% cpu 0.059 total
go 0.00s user 0.00s system 80% cpu 0.010 total
by maccard
12/11/2025 at 6:09:27 PM
It's not interpreting- Python is loading the already byte compiled version. But it's also statting several files (various extensions).I believe in the past people have looked at putting the standard library in a zip file instead of splatted out into a bunch of files in a dirtree. In that case, I think python would just do a few stats, find the zipfile, loaded the whole thing into RAM, and then index into the file.
by dekhn
12/11/2025 at 8:10:54 PM
> In that case, I think python would just do a few stats, find the zipfile, loaded the whole thing into RAM, and then index into the file."If python was implemented totally different it might be fast" - sure, but it's not!
by maccard
12/11/2025 at 8:21:05 PM
No, this feature already exists.by dekhn
12/11/2025 at 8:22:58 PM
Great - how do I use it?by maccard
12/12/2025 at 4:56:06 PM
You should look at the self-executing .pex file format (https://docs.pex-tool.org/whatispex.html). The whole python program exists as a single file. You can also unzip the .pex and inspect the dependency tree.It's tooling agnostic and there are a couple ways to generate them, but the easiest it to just use pants build.
Pants also does dependency traversal (that's the main reason we started using it, deploying a microservices monorepo) so it only packages the necessary modules.
I haven't profiled it yet for cold starts, maybe I'll test that real quick.
https://www.pantsbuild.org/dev/docs/python/overview/pex
Edit: just ran it on a hello world with py3.14 on m3 macbook pro, about 100 +/-30 ms for `python -m hello` and 300-400 (but wild variance) for executing the pex with `./hello/binary.pex`.
I'm not sure if a pants expert could eke out more speed gains and I'm also not sure if this strategy would win out with a lot of dependencies. I'm guessing the time required to stat every imported file pales in comparison to the actual load time, and with pex, everything needs to be unzipped first.
Pex is honestly best when you want to build and distribute an application as a single file (there are flags to bundle the python interpreter too).
The other option is mypyc, though again that seems to mostly speed up runtime https://github.com/mypyc/mypyc
Now if I use `python -S` (disables `import site` on initialization), that gets down to ~15ms execution time for hello world. But that gain gets killed as soon as you start trying to import certain modules (there is a very limited set of modules you can work with and still keep speedup. So if you whole script is pure python with no imports, you could probably have a 20ms cold start).
by kortex
12/11/2025 at 1:12:42 PM
Just a guess - but perhaps the startup time is before `time` is even imported?by tlyleung
12/11/2025 at 1:20:28 PM
`time` is a shell command that you can use to invoke other commands and track their runtime.by williadc
12/11/2025 at 4:03:31 PM
It might not be the fastest but I suspect something weird is happening with python resolution.For instance `uv run` has its own fair share of overhead.
$ hyperfine --warmup 10 -L py "uv run python,~/.local/bin/python3.14,/usr/local/bin/python3.12,~/.local/share/uv/python/pypy-3.11.13-macos-aarch64-none/bin/pypy3.11" "{py} -c 'exit(0)'"
Benchmark 1: uv run python -c 'exit(0)'
Time (mean ± σ): 58.4 ms ± 19.3 ms [User: 26.4 ms, System: 21.7 ms]
Range (min … max): 48.2 ms … 138.0 ms 50 runs
Benchmark 2: ~/.local/bin/python3.14 -c 'exit(0)'
Time (mean ± σ): 13.3 ms ± 6.9 ms [User: 8.0 ms, System: 2.5 ms]
Range (min … max): 9.9 ms … 53.7 ms 174 runs
Benchmark 3: /usr/local/bin/python3.12 -c 'exit(0)'
Time (mean ± σ): 16.4 ms ± 7.6 ms [User: 8.9 ms, System: 3.7 ms]
Range (min … max): 12.2 ms … 65.2 ms 152 runs
Benchmark 4: ~/.local/share/uv/python/pypy-3.11.13-macos-aarch64-none/bin/pypy3.11 -c 'exit(0)'
Time (mean ± σ): 18.6 ms ± 7.4 ms [User: 10.0 ms, System: 5.0 ms]
Range (min … max): 14.4 ms … 63.5 ms 138 runs
Summary
~/.local/bin/python3.14 -c 'exit(0)' ran
1.23 ± 0.86 times faster than /usr/local/bin/python3.12 -c 'exit(0)'
1.40 ± 0.92 times faster than ~/.local/share/uv/python/pypy-3.11.13-macos-aarch64-none/bin/pypy3.11 -c 'exit(0)'
4.40 ± 2.72 times faster than uv run python -c 'exit(0)'by ndr
12/11/2025 at 6:02:12 PM
Run strace on Python starting up- you will see it statting hundreds if not thousands of files. That gets much worse the slower your filesystem is.On my linux system where all the file attributes are cached, it takes about 12ms to completely start, run a pass statement, and exit.
by dekhn
12/11/2025 at 2:10:14 PM
Completely agree on this.Regarding cold-starts, I strongly believe V8 snapshots are perhaps not the best way to achieve fast cold starts with Python (they may be if you are tied to using V8, though!), and will have wide side effects if you go out of the standards packages included on the Pyodide bundle.
To put some perspective: V8 snapshots are storing the whole state of an application (including it's compiled modules). This means that for a Python package that is using Python (one wasm module) + Pydantic-core (one wasm module) + FastAPI... all of those will be included in one snapshot (as well as the application state). This makes sense for browsers, where you want to be able to inspect/recover everything at once.
The issue about this design is that the compiled artifacts and the application state are bundled into one piece artifact (this is not great for AOT designed runtimes, but might be the optimal design for JITs though).
Ideally, you would separate each of the compiled modules from the state of the application. When you do this, you have some advantages: you can deserialize the compiled modules in parallel, and untie the "deserialization" from recovering the state of the application. This design doesn't adapt that well into the V8 architecture (and how it compiles stuff) when JavaScript is the main driver of the execution, however it's ideal when you just use WebAssembly.
This is what we have done at Wasmer, which allows for much faster cold starts than 1 second. Because we cache each of the compiled modules separately, and recover the state of the application later, we can achieve cold-starts that are a magnitude faster than Cloudflare's state of the art (when using pydantic, fastapi and httpx).
If anyone is curious, here is a blogpost where we presented fast-cold starts for the application state (note that the deserialization technique for Wasm modules is applied automatically in Wasmer, and we don't showcase it on the blogpost): https://wasmer.io/posts/announcing-instaboot-instant-cold-st...
Note aside: congrats to the Cloudflare team on their work on Python on Workers, it's inspiring to all providers on the space... keep it up and let's keep challenging the status quo!
by syrusakbary
12/11/2025 at 4:42:54 PM
Big packages shouldn’t be imported until the cli has been parsed, and handed off to main. There’s been work to do this automatically, but it’s good hygiene to avoid it anyway.A modern machine shouldn’t take this long, so likely something big is being imported unnecessarily at startup. If the big package itself is the issue, file it on their tracker.
by mixmastamyk
12/12/2025 at 8:30:29 AM
I don't know why people care so much about a few hundreds of milliseconds for python scripts versus compiled languages that take just ten times less.Real question : what would you do more with the spared time ? You are that in a hurry in your life ?
by az09mugen
12/13/2025 at 8:07:13 AM
It’s not Python runtime startup, but loading all your modules.Use lazy/dynamic imports and you will see it drop .
by paulddraper
12/11/2025 at 1:47:27 PM
Are you comparing the startup time of an interpreted language with the startup time of a compiled language? or you mean that `time python hello.py` > `( time gcc -O2 -o hello hello.c ) && ( time ./hello )` ?by TudorAndrei
12/11/2025 at 1:54:33 PM
I'm referring to the startup time as benchmarked in the following manner: https://github.com/bdrung/startup-timeby randomtoast
12/11/2025 at 1:59:31 PM
Here's the thing - I don't really care if its' because the interpreter has to start up, or there's a remote http call, or we scan the disks for integrity - the end user experience on every run is slower.by maccard
12/11/2025 at 6:45:41 PM
You can run .pyc stuff “directly” with some creativity, and there are some tools to pack “executables” that are just chunked blobs of bytecode.by rcarmo
12/11/2025 at 12:20:44 PM
it depends somewhat on what you import, too. some people would sell their grandmothers to get below 1s when you start importing numpys and scikits.by baq
12/11/2025 at 2:33:37 PM
The upcoming lazy import system may help with startup time…but if the underlying issue wasn’t “Python startup is slow” but rather “a specific program imports modules that take a long time to low”, it’ll only shift the time consumption to runtime.by zbentley
12/12/2025 at 5:44:50 PM
That's totally fine, because many CLI tools are organized like `mytool subcommand --params=a,b...`, and breaking out those subcommands into their own modules and lazy loading everything (which good CLI tools already know to do) means unused code never gets imported.You can already lazy import in python, but the new system makes the syntax sweeter and avoids having to have in-function `import module` calls, which some linters complain about.
by kortex
12/11/2025 at 4:17:28 PM
Reminds me of mercurial cvs!!by dilawar
12/11/2025 at 4:34:46 PM
Yes it's bad enough that there's a chg to (barely) improve the command laten y.(Side note this is why jj is awesome. A `jj log` is almost as fast as `ls`).
by yegle