>>279710171
tl;dr it's mostly the map settings and 40+ users rendering all at once. 
The game barely uses 60% of my gpu on a supersampled index at 144hz in a good map with 5 friends.
No realtime lights, no realtime reflection probe, no render textures, with good occlusion culling, audio import settings correctly set for the audio type, texture import settings optimized and crunched and baked and 20+ users with medium and above rated avatars is not a problem at all, you'll keep your frames.