You mentioned you did a clean install so other than the indexing issue mentioned above I’m not sure either what could be causing that but I will mention something for others that may come across this post:
If you have done an in-place upgrade from a previous version of windows to windows 10 you are not going to get the full performance of windows 10 until you do a clean install after the in-place upgrade. This is because of how windows handles the in-place upgrade and the roll-back files. Long story short, it causes file permission issues where essentially the previous file permissions are fighting with the new file permissions. A clean install after this repairs that.
As for BlackRanger666, I’d suggest turning off indexing for your UE4 directory to see if that is the culprit if you haven’t already. I would have suggested adjusting the systems visual effects for the UI as well but, well, your system should be able to handle petty stuff like that no problem on that hardware. Hopefully disabling indexing of your UE4 directory helps because I can’t think of any other reasonable reason as to why it would be happening in your scenario.