I see that this is infrequently mentioned, and I can say for a fact it solved my performance issues in D2R: Basically, disable core isolation D2R runs smoothly like it does on my Ubuntu machine. It depends on your hardware, so results may vary. If your ram isn’t fast enough, the performance penalty from core isolation can ruin game performance. This was a mid-range DDR4 system.
Windows Security ==> Device Security ==> Core Isolation
The odds that your PC gets hacked if you turn this off is not high, unless you are on shady sites all day.
Maybe it will help someone. Core isolation isn’t mentioned in the standard troubleshooting document. Sometimes people are reinstalling windows for no reason.