For anyone who’s unsure how to read an UEInsight profile, I’ve marked the issue in the shot below.
As you can see, most of the time (i.e. more than 50%), CPU and GPU are just idling, waiting for the RHIT to finish.
You can have a CPU and GPU as fast as you want, if UE doesn’t let it work, it’ll literally render your advanced hardware useless.
The problem does, however, not occur on all machines. In our dev team, only machines with a 3090 RTX are affected. AMD GPUs seem to be immune to the effect. However, this might be down to particular AMD models etc.
But the overall issue is so heavy that I consider the current build of UE as not production ready, even for basic projects. 
Hope someone from Epic sees this and realizes the critical severity of this issue.