I noticed unimported completions were adding 30ms latency to every completion request via imports.GetAllCandidates. It seems to be mostly from allocations:

filepath.Join in findPackage
path.Join in canonicalize
filepath.Rel in canUse
strconv.Atoi in ImportPathToAssumedName (perhaps due to error allocation)

There is also an ioutil.ReadDir call in findPackage that gets called hundreds of time. It was trying to read non-existent directories in the module cache that do exist for different versions of the module.

Maybe the easiest thing is to offer some way to disable unimported package name candidates, leaving unimported package member candidates. That way we wouldn't call GetAllCandidates at all most of the time. I know we don't want config option bloat, though.

Maybe another option is to have a very coarse cache of the result of imports.GetAllCandidates that gets busted whenever any watched file event comes? It's weird that everything is mostly cached already but we still do some much work every time.

/cc @heschik