A few weeks ago I noticed that my PS was getting slower and slower, especially during start up and when saving. After several attempts to solve this problem (change of used RAM etc. etc.) I finally found the reason -the preferences files. Over time they got larger and larger because I had loaded too much into them (many brushes and ,worst, patterns). After saving the brushes, patterns, actions and swatches (and everything else you might need again) I deleted all preferences files. Result: PS fast as the Flash again.
I had to rearrange my workspace after this procedure, but this took only one or two minutes.

Here are the details for the preferences files: Preference filenames and locations in Photoshop CS6

So, if your PS slows down, you might check these files as a possible reason.