I noticed the same thing with my X50v. I upgraded to WM5 and back down a couple of times over the past few months, and then more recently even tried WM6 and then back to 2003 again (had to see what the hype was about).

But ever since the last one (it's been a couple of months), the Axim is noticeably slower. Today plugins take longer to render, programs take longer to load, not a long time, but long enough, a few seconds worth of delay, to be annoying. And the Axim just feels a little glitchier. The other day I had a spontaneous hard reset for no reason, which in all the time I've had it has been a relatively rare happening. So something is definitely off with it.