The culprit is UnrealEditor - there must be a massive memory leak in PCG.
I had one project with PCG open (5.3) and opened a second with PCG open (5.2), system became very, very slow. Crashed the second project by closing all windows from te taskbar, stopped the first one the same way, so there was no UnrealEditor active, according the taskbar. But taskmanager showed still about 50% of memory in use, and resource manager showed the reason:
This machine has 64 Gb internal memory and 3 x 64 GB pagefile on 3 disks.
I know it might be caused by hard-stop Unreal, but my feeling is that is that somewhere in preparing the PCG tasks, there is a huge memory leak. If I restart UnrealEditor again without restarting the machine, it is obvious that this memory is locked for use by other processes and performance is degraded.