P: Layer opacity does not change while moving the slider.

I downgraded thinking I could get back real time change viewing - which I did - but I lost everything custom made I had ever done... gradients, actions, presets etc... I hope you have better luck!

Same with mine - windows 10

I have the same problem with both my PC's running version 21.1.  I've had to downgrade to CC 2019 to revert back to the best working version.  I'm running Windows 10 with both my PC's utilizing high-end Nvidia cards with the latest drivers.  I haven't tested all of the sliders but the shadows/highlights sliders no longer work in real-time either with version 21.1.

Same here. At first I thought it was related to other malfunctions introduced with this version and noticeable when using a scale value of 125% ( https://feedback.photoshop.com/photoshop_family/topics/photoshop-cc-2020-v21-1-0-recessed-layer-thumbnails ), since it also had to do with the Layers palette, but I guess this one is a different issue. Too many problems with this version, going back to 21.0.3.

My opacity slider also does not update in real-time
 while sliding

Horrible bug !!! I updated Photoshop yesterday. Yes, Ctrl+T works for layers containing images, but does not for LUTs. I need 30% time more to adjust images. Does Adobe presume any subscription bonus due to my longer time spent on working with Phostoshop 21.1 ?

If you pres controll+T (transform) then it starts to work correctly.

You should start a different thread and provide more details. (OS/version, whether you're using a table or mouse, etc)

Engineering is looking into the slider/update issue.

Having same issues. Plus curvature pen tool started to act really slow. 
And honestly absolutely no performance improvements noticed.