Did you Upgrade to windows 10 or did you do a clean install? I did a clean install and Windows 10 was running much better when it doesn’t have the old windows data. Does it still lag with just 2 monitors connected? Also what is your PC Spec? Which type of gpu? Are you using SLI? Also try checking for software conflict. I have an EVGA gpu and PrecisionX driver was causing me problems and the same with ASUS Sonic Suite 2 driver so i close them down or uninstall before opening up UE4.