Our CI took 2 minutes to install the requirements. Adding UV dropped that to seconds. Now most time is spent on running tests, instead of installing requirements.
Of course we could've cached the venv, but cache invalidation is hard, and this is a very cheap way to avoid it.
Or you could cache the solve, surely? Simply explicitly including your transitive dependencies and pinning everything should do the trick - but there is work being done actively on a lockfile standard, it's just the sort of topic that attracts ungodly amounts of bikeshedding.
(One of these days I'll have to figure out this "CI" thing. But my main focus is really just solving interesting programming problems, and making simple elegant tools for mostly-small tasks.)
Of course we could've cached the venv, but cache invalidation is hard, and this is a very cheap way to avoid it.