The new FileSet.AddExistingFiles method is quadratic when called repeatedly (as intended), and this is a serious performance problem in gopls that cannot be fixed within gopls, since it cannot change the representation of FileSet. A performance problem of this magnitude is properly considered a bug. (My bug, but still.)

https://go.dev/cl/675736 is a fix: instead of a (repeatedly) sorted array of files, we use a tree. It's 1000x faster on the benchmark that most closely models the problematic case. I propose to merge it into go1.25.

@golang/release