That is so weird… going off fullscreen fixes the lag for me aswell. Having the mouse lagging on the editor issue with UE 4.25.4 built from source. My guess is that it’s having issues utilizing my gpu (6700xt) cause it won’t go over 25% or so… Been messing with my gpu settings for a couple of days now. I once fixed it by enabling vsync but then there are some weird blacking out moments whenever I hover over settings… With the new drivers update as of yesterday vsync is not solving the mouse lagging part anymore but still has those weird screen blacking moments lol.