Windows build importlib performance regression

首先，使用提供的 import_models.py 和 some_class.py 配合 -X importtime，复现 Windows 10 上的计时差异；然后比较执行 python -m compileall . 前后的运行结果，以及使用 -B 时的运行结果。检查加载模块时涉及的 importlib 和 Windows 特定导入路径。完成的标准是确定缓存导入变慢的原因，并通过经过验证的对比记录或修复该回归。

Bug report

I've a file import_models.py:

import time
from ctypes import c_longdouble
from multiprocessing import Value


def import_models_func(_, import_value, exec_value):
    init = time.process_time()
    import some_class
    import_value.value = time.process_time() - init
    exec_value.value = 0


if __name__ == '__main__':
    import_models_func(0, Value(c_longdouble, lock=False), Value(c_longdouble, lock=False))

The some_class.py is:

import time


class SomeClass:
    __slots__ = ('a', 'b', 'c')

    def __init__(self):
        self.a = time.time()
        self.b = self.a / 100
        self.c = self.b ** 2

    B = 2
    for _ in range(10):
        B **= 2

    @property
    def a_val(self):
        return self.a

    @a_val.getter
    def a_val(self):
        return self.a

    @property
    def b_val(self):
        return self.b

    @b_val.getter
    def b_val(self):
        return self.b

    @property
    def c_val(self):
        return self.c

    @c_val.getter
    def c_val(self):
        return self.c

The directory containing both files has no __pycache__. I run python -X importtime import_models.py and receive:

import time:       521 |        521 | some_class

Then I run python -m compileall . and re-run the command above, the output is:

import time:       548 |        548 | some_class

I've tried the sampling with multiprocessing governor, which is run with -B. The governor runs this scenario for 100 times, both mean and median statistics values of the timing measured mostly provide with the same results: the timing without __pycache__ is less than when the source files are pre-compiled. Which is a non-sense keeping in mind what the pre-compiled byte-code cache is built for.

I've searched for Windows-related fixes through the versions 3.9 to 3.12 and found nothing about this regression.

Linux build in a native (non-WSL, non-emulated) environment does not have such an issue (the bytecode-compiled version gets imported faster, up to 10 times, than a raw non-compiled source)

Your environment

• CPython versions tested on: Python 3.9.13 (tags/v3.9.13:6de2ca5, May 17 2022, 16:36:42) [MSC v.1929 64 bit (AMD64)] on win32
• Operating system and architecture: Windows 10 Enterprise 21H2 19044.2251