SSDT is insanely slow rebuilding our database model in the 'background'. And it's clearly tripping over itself somewhere because system CPU and IO is typically near zero while these rebuilds are running. Very annoying.
VS2015 improved SSDT builds, I think. Build times were cut in half, but memory usage skyrocketed. I'm supporting a legacy system that used to be north of an hour to build under SSDT. Now it seems closer to 20 minutes, if it doesn't get "Out of Memory".
I do think it maxes out 1 CPU, it's just single-threaded.
In general, anything that tries to pop-open a GUI and just drags makes me roll my eyes. EDMX files, I'm looking in your direction.