This is a cpu problem. It appears that in some projects Unreal offloads all the GPU calculations to the CPU and that is the reason for these awful performance issues.
Take a look here:Extremely bad performance in a paper2d project that uses streaming levels - World Creation - Epic Developer Community Forums
If you pay close attention to the pictures you can see my CPU usage is out of the roof and my GPU is in idle mode. I hope the Devs will soon figure out what is going on because it is a serious issue and i see that it affects more and more people over time.