I've run into the import issue as well, it's especially painful if something imported is doing work using a slow library (e.g. a config module loading a HOCON config using pyhocon a.k.a slowasmolasseshocon)
That will be another slow down happening at import stage. But even if ideally everything you import only does import with not much work, the import overhead mentioned above still exists. But frankly I’m not sure how significant it is on a typical personal computer with SSD, especially when the OS may cache recently used files already.
The problem you mention can be significant because some design patterns in Python encourages this. Eg function arg with a default that’s expensive. Unfortunately this whole speeds up repeatedly calling the func, the cost is paid at first import.
Unless your 1) execution environment (CPU, dlls) or 2) Python source code files change, you can use “unexec” strategies like Emacs
If you really care about the startup time, you could pre-compile it. Nuitka and Cython could do this with different limitations.