You’ll want to update CS5 to 12.0.4 (via Help->Updates), then grab this plug-in (Mac)/registry key (Win). Photoshop PM Jeff Tranberry writes, “Saving the file can be much faster (20x in some cases), if you have a fast hard disk and enough disk space to hold the larger file size,” and performance expert Lloyd Chambers goes into detail on his site.
http://blogs.adobe.com/jnack/2011/05/save-big-photoshop-files-up-to-20x-faster.html