This is really beyond annoying......

umm, Ive always used dx11 :/

I disabled all mods, as I said above, and issue still persists, it is only one specific slot/window for some reason.
Worked fine for a while then same deal, back to 1-3 fps, even if I set graphics down too low, which Ive never had to do before. And fps was still 1-3.
All 5 windows can run on good easily in past until just other day where it is 1 window/slot that has serious fps issues. Even with ALL mods disabled, even with jamba/isboxer disabled. Very weird.