if even typing is slow, then i would assume that you have too many processes open on your computer... ur processor is trying really hard to deligate work...
i would suggest, closing a few programs ...
after installing flash builder, i have experienced no such problems on my computer...
You would think it may be too many processes, but I do not think so.
I can close down Flash Builder and then bring up Flex3 and things move smoothly. Waiting for an Adobe person to chime in on if I should take it off the machine and re-install.
I've noticed that if you have the Related topics tab in the Help panel open while you do code editing, the IDE can sometimes grind to a halt while it tries to fetch help content for the things you're typing. Apparently the help is not running in it's own thread and so it is blocking the entire application. Workaround is to not have the related topics tab open unless you're really searching for something from the help.