damson As you rightly pointed out, the video immediately drops to 60fps once the Sharpness level is set above 0
This has been a known bug reported many times over for over a year, it got fixed temporarily but came back with an update - Good luck 🙂
Probably needs a new engine to resolve the issue properly.
Too be fair it's not as bad as it used to be, as i used to get framerates of approx 25 fps with sharpness and Ai Passthrough applied, but now get better result of around 65 fps with a refresh rate set at 90 hz.
The main issue is the refresh rate which may not be picked up by some people's brains but i see a strobe effect at 72 hz or lower, so when it switches from 120 hz to 72 hz it's like night and day.
As you know this is a Meta issue and not an issue that SLR can resolve - You were spot on with your analysis IMO 🙂