Re: PSA: Use fullscreen mode, not borderless (performance issue)
@5moothassWallaceI am running the steam version. I have ran the game in windowed, borderless, and in fullscreen. I have not seen the behaviour you mention here. Ive never had a game drop its FPS by half just going to borderless. Every instance Ive ever seen this occur it was due to refresh rate / vsync type issues, where the game was being artificially capped by some process while in window mode. The fact that you say 60fps is telling. 60FPS would be one of those expected capping points where a vsync issue would be at play.
Have you verified the monitor is in fact running at 144hz in the control panel?