Including any LUA coding.
Bombs? Gbombs3 lag like hell when you spawn like 5 smoking bombs… uugh.
Prop spam, Explosive spam, Nuke spam, etc.
Anything, there more there is of something, the lower ya fps will go
Ragdolls. Or even corpses if they are in the thousands.
Playing with more than 3 people and using a ragdoll.
NPCs, especialy online