Bit of an update:
I decided to try T4 on a better machine (i7-4770K, 32gb ram, GeForce GTX 760):
It comes up fine and loads the same files as before without any apparent issues. The RTP button is 'off' initially, so it defaults to the usual view.
However, if I press the RTP button...uh-oh. The system starts to hang as memory shoots almost to max, then everything goes totally unresponsive, virtually frozen for literally 22 minutes. I'd opened Task Manager so I could see what was happening, and oddly the cpu usage goes to -zero- and memory was suddenly down to 71mb (yes, mb), all frozen for about 5 minutes. Then memory went to around 15gb with cpu at about 20%, but things are still frozen and stayed about the same for more minutes with some small changes in mem/cpu, Finally, at about 22 minutes, everything came to life and the RTP preview began to render (and it's very nice if I may say so!)
After that things seemed workable, and I can change to/from RTP without anything but a little delay while it updates. Also, I rendered an image to test that part, and it came through in about 52 minutes (which is great imho since it takes 2.25 hours on the other machine, though I don't know how much to credit solely to the faster cpu and more RAM. Still, much better.)
Unfortunately, if I close down and restart everything and touch RTP, the 20 minute freeze happens all over again. Frustrating at best..
I'll test T3 on this machine and see how fast the same scene renders and will report back shortly.