The above mentioned Python’s import delay. This is a notorious behavior in high performance situation (including your cli startup time example). Essentially the import process is IO intensive. This comes up in massive parallel applications as at launch each process is importing the same stuffs. It is briefly mentioned for example in https://docs.nersc.gov/development/languages/python/

If you really care about the startup time, you could pre-compile it. Nuitka and Cython could do this with different limitations.