When Photoshop saves a file, it creates a preview of the flattened image, plus an icon for the desktop. You'll see the progress bar crawl across the screen while it does so. Sometimes this part of the save process takes longer than the actual "saving." So for interim saves while you work, just turn off the visibility of all your layers before you save. Photoshop will virtually skip the preview building process (as all it has to do is create a blank preview), and jump right to the file writing process. Tah dah! That's it.
Lots of layers and layer groups? No problem, create one master group that contains all the other layers and groups. Then it's just one click on the master group's visibility icon to hide all the layers. When you're done working, just leave the layers visible for the last save, so Photoshop can generate the previews.
There are Photoshop preferences that control the preview builiding process as well, which you might prefer. However, saving the final version of a file without a preview is a bad habit, as the preview is used by a slew of programs, including the Finder, and comes in very handy. If you're working on a very large file, this can save you as much as a half hour a day, or more.

