Photoshop so laggy with large files, even with monster workstation

My goodness.  That is some spec!

_Two_ RTX 2080Ti ?  Photoshop does not like multiple GPUs.

Is one of them showing in Preferences Performance?

What are the advanced settings?  i.e. are you able to select OpenCL, and what Drawing Mode are you using?

Photoshop graphics processor (GPU) card FAQ

What clock speed is the CPU running at?

Most Photoshop functions are lightly threaded, and clock speed is king.  Check out the Puget Systems articles to see what sort of system spec works best for Photoshop.

What are your Preferences > Performance > Cache settings? 

These can make a significant difference according to the sort of document specs you are working on.  I am not at home right now, and don't have access to Photoshop, so I can't be more specific. 

What zoom ratio are you working at?  My own system uses 7900X which I allow to clock to 4.2Ghz; two 960PRO NVMe drives (plus a bunch of other SSDs and other drives) 64Gb of G-Skill 3600 RAM, and GTX 970.  (the GPU comes from my old 3930K system which I did not want to update with new NVIDEA GPUs on the way.  I was expecting amazing performance, and was disappointed.  One of the things that makes a difference is zoom ratio.  It lags at 100% zoom, but is fine at any other zoom ratio.  Some people report having the rulers turned on can slow things down, and I have seen reports of some tablet setting causing problems.

I expect you are using Windows 10, so open Task Manager > Performance > Resource Monitor, and look for bottlenecks.  Be especially careful with the CPU because if Photoshop is only using one core and two threads, it might show very light CPU usage, but it is in fact using 100% of the resource it has assigned to itself.

I honestly can't imagine you having any Scratch file issues, but check the root directory of the primary Scratch drive anyway.  Look for Photoshop temp files, and see how large they are.  I can see 50Gb scratch files without all the RAM I have allowed Photoshop being used.  Even an NVMe drive can take a moment to sift through 50Gb!

What is the make up of the documents causing your problems?  I have a strong feeling that nested Smart Objects with vector layers slows things down.  I'd love to know how Photoshop handles these objects. Where they live, and how they are accessed etc.?

I'm guessing that a spec like yours is designed for NLE > PremPro and AE, or 3D?  My system stands me at >NZ$10K, so your's must have come to serious money.

Hi. Are you sure you're using the most recent GPU drivers available? Does the same thing happen when you test using the Graphics Processor setting, and the variations under "Advanced"? You've got very powerful GPU's but perhaps they're not particularly compatible with Photoshop's GPU requirements (yours isn't listed as a "Tested" GPU on their list). I suggest testing with variations on the GPU settings and determining what works best.

Perhaps review this document for their information on the GPU/driver issue: Photoshop system requirements