I think usually it just runs the compiler in a syntax-check mode. For instance, it runs gcc with -fsyntax-only. I think I have sometimes seen style warnings too, so maybe there is more than that. Maybe I could configure it better to be faster, but for now it doesn't bother me often enough.
Is this for c++? You might benefit from moving to cmake with the ninja backend (ninja being the important part not cmake) -- depending on what kind of projects etc etc.
