Pip is a widely-used, complex Python package installer program with a lot of legacy baggage. The Python Software Foundation recognizes the critical need to update Pip, putting $116K to sponsor a senior dev to modernize Pip.
pip install fails to realize a .whl binary wheel is available.
Thus pip tries to download and install a package from source code.
In the case of a large package like SciPy, OpenCV or Pillow on an embedded system like the Raspberry Pi Zero,
it could take hours or even days to compile, probably failing numerous times due to missing prerequisite binary libraries.
Manual install wheel
A workaround to Pip not automatically finding the desired .whl binary wheel is to download it manually and install the .whl directly. The binary wheels are often available at PyPi from the package download page, for example SciPy. For embedded systems such as Raspberry Pi, there may be non-PyPi sites such as PiWheels.
Download the file, then pip install from the file like:
If the wheel binary is not compatible with the system, it will fail to import or run.
In that case, simply
pip uninstall my_package_name and try something else.