I downloaded a few packages from Project AI's website, installed them, and to my surprise there is very little traffic at major airports, I went to KBOS and there were only 10 planes on the tarmac, with the UA, CO, DL (mainline, express and shuttle) and US packages installed. I Also installed a AFCAD file for KBOS with up to 80 gates if I remember correctly. Both traffic settings (ATC and dynamic scenery) in FS are set to max.
It was working properly before I formatted my HD, all airports were filled, but now it's rather dead. I had much more traffic before with the same packages installed.