I just did a test to see if it’s even an Unreal Engine 4 problem, as I have now gone from windows 7 to windows 10 and have the exact same problem.
I disabled 2 of my monitors (I have 3, like BlackRang666), and suddenly, UE4 runs fine again.
I noticed that there was a similar lag in my task bar in windows when hovering over something like the google chrome browser; the animation for bringing up a preview of the window had some kind of lag in it.
So then I tried just 2 monitors, and the performance dropped massively, but not as bad as having all 3 monitors running.
I then checked this out online and found other people with the exact same problem within windows using multiple displays.
The thing that makes me confused however is that I’ve been running 3 monitors for years without a hitch, now suddenly, it’s not running so great.
I don’t think Epic Games can help with this issue as it is related to windows, not UE4, however this points us in the right direction and gives us something to go off to try and find a solution.
Out of curiosity, I moved my main UE4 window to my left most monitor, and suddenly it’s all smooth again, but if I move any of the UE4 windows to the primary or right most monitor, constant lag,
Having done a bit of research, and remembering something, my primary monitor was set to 59hz before the reformat, where as the other 2 were at 60hz…now Nvidia control panel only lets me choose 60hz in windows 10…