Does the cpu or gpu get affected when server population is high?

[Acidic Blade]

So when i play rust or arma with about 40 people i get 50-60 fps but when theres like 100-200 people i get about 20 fps. What gets affected with the more people there are? Cpu or Gpu?

 

[okcnaline]

CPU gets affected for server population. GPU gets affected the more explosions there are.