I just tried the demo for Twixtor on Final Cut 5. The Mac was not as powerful as the ones I use at work, but still the render times seemed ridiculous. In general, are long render times for this plugin common? If so, I can deal with it, I just need to prepare my renders accordingly.
If you are getting 1 or so seconds computation per frame (30 times slower than real time) then what you are seeing is normal.
As to the Pro version, if you don't get fancy with plugins then I suggest just sticking with the regular version. The Pro version allows you to do things like provide mattes for foreground objects (that you must make by hand).... If you don't do a lot of manual work on images, then there is no need to buy the Pro version.