When does "pip install" build a wheel?

This depends on whether your package is a pure python package (without the need to compile anything, just copy the files somewhere) or a package which also includes c source code (in which case a compilation is necessary and a compiler is called and executed, which takes longer).

http://pythonwheels.com/

You may also want to have a look at the wheel docu:

http://wheel.readthedocs.org/en/latest/

I got the answer, it is just the first time that the wheel will be build, after that, it will read from cache

Today I encountered a problem where a package wasn't being installed properly because it turns out that its build process generates incorrect wheel packages, even though direct installation works just fine.

I did a bit of poking around, and it turns out that as of now (pip == 8.1.2), there isn't a direct way to control whether or not pip will try to build a wheel out of a given package. I found the relevant source code, and apparently, the wheel build process is used if and only if:

• the wheel module is importable
• a cache directory is in use

As a result of that logic, one can indirectly disable pip's use of wheel-based builds by passing --no-cache-dir on the install command line.