Here is a thought. One the bottom left of photoshop there is a pull down menu that gives you the ability to see the image dimensions, size etc. Select size. It will show something like 50m/50M to start. Then asy you do things, like focus stacking, each time you do, you will see your size increase dramatically. I us Nik plugins, and if I use a few, the size of the picture grows dramatically, up to 700mb, to ove 3 gb's in one instance. So now you are working with multiple layer. We he D850 image size, or worse yet a tiff file, your image size can get huge. Now save the file. It could be way bigger than the original file.
What you can do is flatten the image after you are done, and the file size will drop drastically. What I do is denoise first, flatten the image, then do something else which might create a new layer, then flatten. Eache time you flatten, you will keep the changes you made with a much smaller file.

I do panoramas. If you use say 5 picture, blend them, you will have layers, one for each picture, plus the final blended picture. Now you have many gb's in RAM. Since you have finished with the picture layer, dlete their layers and close the individual pictures.

I blended 13 pictures, in a pano the other day, it took 3 hours for the final product. It was 6 gb in size

If you focus stack, the same thing will happen, you ile size will grow dramatically as you have more and more layers. Try flattening the image while and after completing the process. you my help youe slowness issues.
Of course oncw you flatten, you cannot go back and modify the layers.