Optimizing cache levels and tile size for huge pixel dimensions AND many layers

Thank you everyone for the advice and tips! I've been experimenting with different settings and am finding the only thing that really helps is increase the memory size, but I think it could be because the files are just way too big 😞

I'm having to make gigantic extensions and convert to 16bit to fix gradients at the end so there is just alot of loading. 

That's a very clear and concise breakdown, thanks Brett.

What jumps out in the OPs settings is the number of history states. 200 is going to kill you. Take it down to 20.

I'd also reduce the memory allocation to 60% or so. Remember, this is Apple silicon, where the GPU eats into system RAM, and the GPU will use a lot.

Which leaves the most important thing: the scratch disk. This is where the heavy lifting is. You'll need at least 500 GB to be reasonably safe, but personally I wouldn't even begin this with less than 1TB. It should be on a fast internal drive (NVMe) - if you have to resort to external drives for scratch, things will slow down dramatically.

As with any performance settings, you end up giving up performance in one area to gain it somewhere else. You are never removing work which needs to be done, just determining when and how it is done. The optimal settings are determined by the types of work you are doing, some of that is characterized by the composition of your file and also what tools you use. So a little more context may help here.

Memory Usage: This determines the size of the Scratch Disk file that gets created when Photoshop is launched. This is a tempory file which acts as additional RAM. When you increase this setting, you are choosing for PS to launch more slowly while it creates a bigger file. When you decrease, while the launch is faster, the slowdown will happen later when PS needs to increase the size of the file on the fly because the work you are doing requires additional space. This takes longer to do once PS is up and running which is why the default is so high. If you know you are doing high memory work, keep this value up. If you are working with small files, keep it low. 

History States: Obviously, this is the number of Undo's you can do. But remember, in order for Photoshop to remember all those document states, it is storing another full copy of your file in memory (RAM/Scratch Disk). This is absolutely the largest memory hog you will encounter. If you are working with very large files, you'll want to keep this value lower. 

Cache Levels: This is used for zooming in and out of your image. It pre-caches your image at different resolutions so that PS can quickly go from where you are to another zoom. The first is always whatever your current view is. If you have it set to only 1, it means every time you zoom in or out, PS has to re-write the entire file to memory for that size. For every one you add, a new level of zoom is memorized (in RAM/Scratch Disk) so that the change in magnification is sped up. The trade off is: higher levels will cause PS will take longer to open files to create the caches and take up a bunch of memory, while lower levels take up less memory but has to redraw the file more often. 

Tile Size: Because the display has to be refeshed when you make a change, PS divides files into chuncks. That way when you make an edit, you don't have to redraw the whole image. If you are doing full document edits (filters, adjustments, and the like) then you want to have larger tiles, that way PS is redrawing fewer tiles overall. If you are doing more descreet edits (ie small brush strokes), you want to use smaller tile sizes, because only the tiles that the brush touches will get redrawn.

Not sure if you've seen this support article but it has some helpful info in there in addition to talking about cache levels and stuff: https://helpx.adobe.com/photoshop/kb/optimize-photoshop-cc-performance.html

Based on this it seems like you might want to try just setting cache levels higher:

If you use files with larger pixel dimensions—say, 50 megapixels or larger—set Cache Levels higher than 4. Higher cache levels speed up redrawing.

It doesn't mention anything about number of layers like it does in the app so that's probably better. I've personally always just kept the cache level set to 8, not sure how much of a difference it truly makes.

I will say that history states might make a bigger difference than you expect - especially for huge files, you can see the difference after a while if you clear the history, a lot lagginess goes away if you had a lot of states. Until you build up more again of course.

The article also talks about other things that might be helpful like the "efficiency" indicator.