I agree with you totally, and I will also buy the game regardless. However, I suspect the release version will be a little different from the demo. If not, I would expect a patch soon after release. At the very least I expect frame rates to be smoothed out a little. My machine is a bit more powerfull than what you described and I experience the same thing as you. What's strange to me is that it doesn't even matter how I set the graphics and sound options. I get about the same performance at highest or lowest settings and there are occasional slowdowns that don't seem to happen for any reason. I could understand if there were tons of units on screen, or new sound/music files being loaded, but I don't see any hard drive access during slow-down periods. I suspect it's related to the AI and/or user interface because after a retreat is started it speeds way up, so it's not graphics or sound because they are still there.
I haven't found any good way to fix the problem, so I sit in quiet frustration the same as you. Look on the bright side. At least it's better than Force Comander. That had to be one of the worst UI's in game history.
Gary Swift (aka gswift)
Lenexa, Kansas USA
You can try to monkey-proof your program, but you'll never monkey-proof the monkey.