Maybe it is something related to the Operating system thread administration. Can you try some 78 or 79 threads instead?
Maybe the CPU gets heater and it reduce the frequency. Check first running it with less than 80 thread try some 70, 75... Just to discard some cases.
Ubuntu 23.04
I have tried 78, 50, 55 all the same results. Anything over 40 decreases the speed by half with twice the threads. Maybe a Kernel issue?