The specifics are kind of important.
What are the pixel dimensions? Bit depth? How many layers? What file format - PSD? How big do the files end up on disk?
If PSD, there's a setting in the preferences that allows you to turn off the "flate" compression, so if you have plenty of disk space you could save without compression - that can sometimes be MUCH faster.
By the way 6 GB is not a lot of RAM by today's standards, and it's not terribly expensive. Not sure what your budget is like, but if you really are pushing up against the limits of your RAM (quite possible if you're editing large documents), look into an upgrade to 16 GB or more.
Yes, I could add more RAM, and I'll probably do that today.
I restarted the machine and my performance is normal again. Why didn't I restart sooner? I had JUST started the machine, launched PS and tried opening a file or two. The performance was terrible, and it's not like the machine had been running all day with heavy use. It was basically a "fresh start."
I still don't understand what caused this performance lag, since the machine had JUST been started.
The files in question were native Canon 5D dimensions, uncropped, several layers, PSD format, 140 Mb. It was taking several minutes to both OPEN and SAVE this file again - highly unusual for this or any previous machine I've owned.
I'd suggest looking at the system logs to ensure you don't have a disk problem brewing. I can't help you with the specifics as I'm a PC expert. I know Windows logs errors if it gets controller timeouts or has to retry disk reads to get valid data; I suspect OSX does too.
The problem with things that just go away is that they always seem to just come back. I hope that's not the case for you.