I too had a 2 fps issue on some (MHrise and Last Epoch, but not stellaris) games too. But when the game fullscreen wasn't active, it was full fps. Also when disabling freesync, game was full fps when it was active fullscreen, but went to unlimited fps when not active.
Found out that the AMD adrenaline fps limiter was set to 15 (minimum), changed that and it seemed to fix this. Perhaps it was set to even lower as I couldn't get back to 2 fps.