So sometimes when you start to get a large file with hundreds of layers photoshop can start to slow down a bit, even if you have a great machine.
To speed up things, it would be great if you could essentially crop a selection or "selections" of your document, and continue to work while retaining all your layers. You would mark "Focus Areas". Your focus area would crop that area off into a new document when double clicked, while retaining all the same layers of the Parent document it came from. Its essentially like a mix of crop tool, and smart objects. You can create multiple focus areas, and even up rez your Focused Area documents to get more pixels to work with as well. When your done you save, and photoshop will automatically update the focused area, and all the layers will link/merge directly back their original file. Again similar to smart objects.
Benefits of this:
1.) Divide your large Photoshop Document into smaller bite size areas when needed to speed up photoshop while RETAINING layers
2.) Can work on areas that are too small in pixels resolution from original file, by Up rezing them in a Focused Area linked file(like smart objects)
Similar but Not the Same as:
1.) Croping - Cropping an area non destructively , hides the outside work but its still there. I could be wrong but i beleive it doesnt actually speed up your file, everything is still there, so all that luggage in the background is still being carried.
2.) Copying and pasting a marque selection of the image to a new document- You can do this , and upres this if you like, but the problem here is you lose your layers, have to rememeber your original pasted document size if you rescale, and you must keep your originally file open to paste it back into the marque selection if you want an accurate position.