This may not always be a desirable behavior, for use cases where detecting errors early is desirable, such as in build farms. The expected symptom is a slowdown, the size of which is hard to estimate without real world testing.
Yet, lacking this information, I do expect that this PR is a net improvement.

For build farms: do they substitute? I'm not sure builders in a build farm do substitute.

ie: I think the controlling system does the substitution on their behalf.

https://github.com/NixOS/hydra/blob/5dd4736aedc1d1356c54bd737db813e25192a885/src/hydra-queue-runner/build-remote.cc#L92-L98

when asking a build slave what it has, you give it a boolean, saying if you will allow it to check the cache

so the queryValidPaths function may infact trigger large amounts of network traffic

Nix has a --no-net flag in master, so closing.