I'm exactly in the same situation, I'm using QuickSync encoder, my game run at 45-60fps but the stream (720p30fps) drop to 20-25 fps at, iGPU is not overloaded by the compression (40%), my CPU is fine too (70% dedicate to the game exclusively).
(Here my post for more details :
https://obsproject.com/forum/thread...s-encoding-while-1080p-48-50fps-ingame.68011/)
Most of people here (lots of thread talking about it last days) have a problem with OBS : Game running fine, nothing is overloaded but stream drop fps.
There is something to know, when my game is 48fps, stream down to 24fps (double). I think something in last version of OBS just divide fps in order to get the goal of your selected fps (in my case 30fps) , there is no compensation by the encoder.
When i'm over 60fps, i get solid 30fps, when i'm 48-50fps, the encoder have difficulty to select 30fps of it, it just divide fps by 2.
Maybe OBS need more cache or delay (400ms isn't enough ?) to get the time to take 30 fps constant of an inconstant higher framerate.
Wait and see.