I7 6700 16 GB RAM and 512 SSD having the same issue. Super dirty and old Windows 10 installation running 4 different web browsers with tons of tabs (have some troubles focusing LOL), and I just wanted to say that this trick did it for me. After I added the " -DX12" (without the quotes) after the target in the shortcut that invokes the Unreal engine (I just got installed the UE 4.27), the slowness went away.
All is working great now 
Thank you!