why is my MacBook Pro underperforming since I have updated it to high Sierra

before the update I was using Sierra and that would run some games with acceptable frame rates but now since I updated I'm getting real bad fps issues on most gpu demanding programs

I was getting 100 fps on cs:go but now I'm getting 20-10 fps on it thats just one example

I know Macs aren't made for gaming but I don't care because I have a windows pc but its still fun to be able to play some games when bored on my Mac