The Mac Performance Guide from Lloyd Chambers has a post on a new plugin that can dramatically cut the time it takes Photoshop to save a very large (1GB and up) layered image file. 

The plugin, called DisallowFlateCompressedPSD.plugin, disables the compression that is normally mandatory when saving layered PSD/PSB files. This greatly reduces save time at the cost of larger file sizes. 

In one test that Chambers conducted of a Mac Pro with a three-drive SSD RAID, the new plugin cut the save time for a 2.88GB PSD file from 130 seconds to 8 seconds.

 Found via John Nack on Adobe