Speeding up large directory pip installs

[Peter Baumgartner]

I'm interested in helping to resolve https://github.com/pypa/pip/issues/2195. I put together some local benchmarks against pip 9.03 and 10.0.0b2 at https://gist.github.com/ipmb/2332d9bd3d6a70ab509f276fa346d534. The project I'm using is small, but enough to exhibit the problem.

Building and installing a wheel takes <2s, but installing from the directory takes >10s. With the new build system stuff starting to land in pip 10, what is the path forward to faster directory installs? If somebody can point me in the right direction, I'd love to contribute :)