Grid and Guides causing slowdown

I have the same issue. When I turn on the grid, Photoshop becomes virtually unusable.

Yesterday I decided to update my Photoshop 21 to see if Adobe's amazing software engineers had solved its performance issues. I updated to version 21.2.3 (the current version)

Nope. Once again, I need to avoid the current version of Photoshop in order to get serious work done. So glad I'm paying monthly for this. I should stop being an idiot and cancel.

To address Nancy_OShea's questions:

"Which operating system?"

Windows 10. I keep it up to date.

"How much free HD space do you have? "

1.5TB via my Photoshop preferences scratch disk settings.

This including a physical 1TB stand-alone scratch disk HD which is specified as my primary scratch disk.

"And how much RAM are you allocating to Photoshop?"

My sytem has 64GB of DDR4. Even if I tell Photoshop it can use 100% of it, it makes no difference.

I also tried unchecking "Anti-alias Guides and Paths" in advanced performance settings.

None of this has anything to do with the issue.

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Here's what I think happened - Photoshop team reworked how grids and guides are drawn, likely in an attempt to optimize them, but it appears their update just made things much much worse.

A quick visual analysis - 

The 3 following images are screenshots of a 1200x800 document in Photoshop 21.2.3 with the grid turned on.

Gridline every: 512px, Subdivisions: 16.

^ Notice the randomness of the dotted lines. Why do they look like this?

If I zoom in, in Photoshop, to one of the grid squares it looks like this: The linestyle is ridiculously bad. Looks glitched.

NOW, if I screenshot one of the grid intersections and blow it up I see this:

What is this??? How many lines are being drawn on top of each other? What is this supposed to be???

I'd say something went wrong.

Is Photoshop duplicating lines, calling numerous line drawing functions unnecessarily?

(Remember that time a few years back when the Photoshop team broke the rectangular marquee tool and left it broken for a month?)

I closed Photoshop 21.2.3. I opened Photoshop 20.0.1 and did the same thing - 

The 3 following images are screenshots of a 1200x800 document in Photoshop 20.0.1 with the grid turned on.

Gridline every: 512px, Subdivisions: 16.

Looks very different. Grids changed in Photoshop 21.

Now I'll zoom into one square again:

Looks as expected.

Let's look closely at a grid intersection:

Perfect. Dotted lines. As expected.

___

Conclusion: Photoshop 21's grids (and guides?) are drawn with less effectively optimized methods than previous versions so the Photoshop team can maintain their incompetence quota. Go team!

I am also having this issue. It wasn't a problem a few months ago. 

EDIT: I've installed the earliest version I could (20.0.10) and that is working now as a workaround.

Which operating system?

How much free HD space do you have? 

And how much RAM are you allocating to Photoshop?