Little insight on what I did. I timed depguard's Run method using depguard's cli alone. Before the fix it took on average a minute to go through golangci-lint repository. This does not include the time it took to build the loader. With the new fix in place it takes on average 3 milliseconds.

Before it was using go's build's context and the Import method on that. This killed time as I think it was doing a lot more than what I really needed. Instead, I am using build's context GOROOT and running iotuil.ReadDir to get the prefix for all known root packages. And if there is one thing depguard was good at, it was efficiently looking up prefixes from a slice.

Would love to hear your numbers as well.