Mm, I found this (lockfile having perf implications for dependencies) scouring the source and backtracking to the docs got me to the FYI
https://www.pantsbuild.org/docs/python-third-party-dependencies#lockfiles here.
The lockfile does speed it up a fair bit, which is nice! I started out without it because we've maintained our own "lockfile" via pip-compile and bash, and I didnt realize the benefits of a (pex) lockfile vs a single requirements file pointing out exact dependencies for every source (pip-compile result).
We're still suffering from scoped pytest fixtures re-computing for every test file (its aws-cdk templates which are SLOW to generate). Thankfully they are a realitively small subset of the module.
Ill follow up this thread when I have tested the other option, although the docs seem tell me not to now that I use a lockfile :)