It seems like the performance benchmark suite is broken since pip 10 has been released (April 2018). The previous release was performance 0.6.1.
By the way, https://speed.python.org/ seems to be down :-(
Version 0.7.0 changes:
Fix issue #40 for pip 10 and newer: Remove indirect dependencies. Indirect dependencies were used to install cffi, but Mercurial 4.0 doesn't depend on cffi anymore.