That is quite odd. So the only time you are experiencing the slowed performance is upon initial start up of the project, but if you re-open it, it runs fine?
To utilize the DX12 features in Unreal Engine you need to add the -DX12 to the command line when running UE4 from the UE4Editor.exe. Keep in mind, this is an experimental and new implementation so it might not work as expected. We are working to improve its performance.
Thanks,
Thanks,