Since he pinged me here - FYI RHI stalls were happening for me on 5.3 and 5.4, and significant performance loss as a result. The linked thread shows some of the context.
However I have not been able to test in 5.6 yet to see if there is any improvement. Basically the issue was GameThread running asynchronously with RHIT, so despite operating on separate threads they were still running basically as if in sequence and not in parallel and costing frame time.
The GameThread would idle on waitfortask while the RHIT stalled and add 8-9ms to the total frame time (massive cpu cycle waste and it basically doubled the frame rate on some rigs). I had two systems with the same CPU (i9 12900k) and different GPU, the RTX4080 system was affected more than the RTX3080 system.
There were no RHI warnings or errors in the logs.