Try running OBS or antimicro in the background and see if that helps. Having either running gives me the same great performance as I had in Win7. Launching from Steam Big Picture with a non-steam shortcut works too.
The antimicro solution is weird because I tested it using a blank controller profile and auto profile disabled (so it doesn’t use any CPU detecting applications or sending inputs). I looked at the task manager while launching and running RetroArch and didn’t see any CPU usage from antimicro. But with antimicro running RetroArch uses around 32% CPU running Aria of Sorrow’s title screen in mGBA. With antimicro off, it’s around 27% and has audio crackling and stuttering. I have the High Performance power profile selected in Win10 as well.