I was thinking in terms of CPU usage rather than RAM or GPU performance. Sounds like a high end system though, so together with your wired connection, I guess it is back to the drawing board. I'll keep think about it.
I guess you could open the Task manager Performance tab and see if the thick strokes coincide with maxed out CPU, but my system (i7-3930k at 4Ghz, GTX570, and 32Gb RAM) never comes close to maxing out the CPU or anything else running Photoshop, so I suspect you'll find much the same.