So, this happened again after alt-tabbing for a few seconds. I tried everything including restarting my PC but my FPS now drops to 100 once I log into any of my characters (from 154 in the menus). I measured my GPU temperature right after the drop, MSI Afterburner showed 48 degrees, and my CPU temperature was 53 degrees at that moment. This happens after alt-tabbing for the vast majority of the time, and the problem is that it just keeps capping itself at 100 for a while.
Edt: If this really is an overheating issue then my bad. But that really doesn't seem to be the reason. When this 100 fps thing happens, my fps stays strictly at 100 with virtually no variance. If it were overheating the fps would vary more and would sometimes show 102 103 or something, at least when docked. Also, as we can see the cap comes into action after a player logs any of his chars. My fps in the menus stays the same after this 100 thing (154/180 depending if I am also running RTSS) but drops to 100 just after I log a character, which suggests it is the cap limiting it to 100 for an unknown reason.