I've had Photoshop CS5 for some years now and it's been fine, but recently, maybe a month or so ago, it's started running a -lot- slower, to the point where I'll have to wait ~30 seconds for my brush strokes to show up; this only gets worse as time goes on. There haven't been any hardware changes, or software upgrades/updates on my end between that time, and my graphics card drivers are all up to date. I thought it was my ram, and maybe the file was too large, but it's only using a little less than 4 out of 8 gb ram. Closing all other programs doesn't seem to help, nor does allowing Photoshop to use more ram, via preferences (I went as high as 90%, and nothing helped). Trying to run it in fullscreen causes quite a but of lag as well, but for some reason, that's always happened for me.
Any help? I don't know if my computer specs are going to be of much help, but:
OS Windows 7 x64
Processor: AMD A8-3800 APU with Radeon HD Graphics (4 CPUs), ~2.4GHz