low fps 2080ti

Sooo.. this is gonna be a long one. I came across an old 2080 Ti of my friend, who recently upgraded his rig and asked me to look at his old GPU. When he used it (850W PSU dunno the brand, with a 3600x at that time) it worked as it should for about 2 years or so. Then it started going downhill...

hello everyone I got a pc with a 2080ti, i7 8700k, 32gb 3200mhz and a custom loop but when i play rainbow 6 siege on low settings I will only get like 170 fps and that is low for a 2080ti because I got some friends with lower end graphic cards and play on higher settings and they will get like...