So I enabled the video card for F@H on the 1950x and set cpu to 98% for BOINC. It then did one 16cu unit, and its doing it in 14 hours, like the 2990wx is, but only one at a time.
You could reduce the thread count per PrimeGrid task, such that two tasks fit again.
Or you could actually leave BOINC at 100%. The PrimeGrid tasks with their relatively high thread count won't use each hardware thread for the full time anyway, because they repeatedly need to wait for RAM accesses or for synchronization between program threads.
However, I suspect the best option for running PrimeGrid in parallel with Folding@Home (one or more GPU slots, no CPU slot of course) is to set BOINC to use 50% of the logical CPUs, assuming SMT is enabled in the BIOS, as per default. (BTW, even if you wouldn't want to run a GPU application along with PrimeGrid, PrimeGrid's LLR-based applications would only profit very little from SMT. There may even be cases in which it does not profit from SMT or HT, or in which HT is even somewhat detrimental.)
Though whatever you do, expect both applications to decrease performance of the other to some degree. PrimeGrid's LLR-based applications are heavy on the vector units (and thereby may decrease processor clocks, depending on the CPU) and heavily use the processor caches, the infinity fabric, and the memory controllers. And I presume F@H's GPU application depends a lot on the latter three too.
The upshot is, a F@H + WCG combo usually works out better for F@H performance than a F@H + PG combo. (Well, there are now less than 2 days left to go in the PG challenge…)
