Installed 21.0.2

[DavePinMinn]

PC, Windows 10 Pro, 64-bit.  Photoshop 21.0.2 today.  I7-5820@4.2GHz, 32GB memory, 970 GTX GPU, Dell 3818 4K, 3840x1600 display, scaling at 200%.  Other than the update (and whatever Windows shoves on the system whenever it wants to), NOTHING has been changed on the box in quite a while 'cause it WAS working. 

I'm always a little leery of CC updates to Photoshop or Lightroom, but it USUALLY doesn't cause major problems.  I haven't had to do any heavy editing (Photoshop) in the last couple months, so I'm not sure what the last version was that worked, but the update today to 21.0.2 has caused some odd things.  I did a search for "selection outline flickering" and didn't find anything, but with the GPU on, I do a selection, doesn't matter what with.  Zoom in to anything over 100% to make final selection adjustments (could be 136% or 200% or whatever).  The marching ants flicker on and off, quickly but not in a pattern - not the area selected, but the selection outline.  I"m getting a HUGE lag doing anything in Photoshop, and the selection tool once the selection is flickering is essentially unusable.

Turned the GPU off and it doesn't appear to make any difference.  Still flickers and everything is STILL VERY sluggish.

I put 21.0.0 on and it acts the same way.

At the moment I've got CC installing the oldest version I can get, which appears to be 20.0.8 (CC 2019 appears to be unavailable) on the box, 'cause at the moment Photoshop is barely usable. 

Do I need to set something different on the Graphics Processing - Advanced settings?  I haven't changed ANYTHING in there for a while, so it's still set to

Advanced for Drawing Mode

Use Graphics Processor is checked

Use Open CL is checked

Anti-alias Guides and Paths is checked

30 Bit Display is nto checked.

Is there some problem with 21.? using a 4K monitor?  I looked for anything on PS 2020 having a problem with higher resolution displays, but I figure the 4K displays have been out so long it should have been handled a long time ago...

Anybody have any reasonable (that means, no, I'm not buying a 2080 GPU) suggestions?

[sewamobiljogja2020]

anybody could give suggestion, as the same condition occured to me

[DavePinMinn]

I looked at memory and other things I could think of.  There's a big chunk (something over 15GB) that's "Standby Memory", but when I looked at the Windows forum it said that's just available for programs to use and it's not a problem.  Whatever's going on, both Photoshop and Lightroom are laggy.

And selections in Photoshop are a mess with the blinking marching ants and severe lag.

Is there something going on with the current Photoshop that's causing the bizarre behavior?  I found a discussion way back in 2015, talking about "Experimental features scaliing 200%", but I figure there's no way THAT can still be a problem.  So is there some Photoshop setting I need to change?  Or display setting?

While chasing this I found a reference to scaling UI in the Photoshop preferences.  It was set to Auto, and had the option of 100% or 200%.  I tried both.  Made no difference.

I've shot a 5-6 second piece of video from a cell phone showing the selection sitting there flickering, even though absolutely nothing (as far as I know) is going on.  No mouse movement, no keyboard.  I don't know if it'll help or not, but I tried to insert it here but the preview keeps complaining about the URL.  This is how I displayed it from the website:  www.dperezphoto.com/20200120_195313.mp4

I know this was working normally after installing the 4K monitor so I'm not sure what's changed other than the versions of Photoshop and Lightroom, and, of course, whatever Microsoft dumped on us.