I'm running Flash Builder on a Windows Vista machine. I created a default application project and created a new component. I started editing that new component in the MXML editor, opened the help window to look up some stuff, when back to edit and it freezes up for about 10 seconds. I edited a little more and then it freezes up again for 10 seconds. I looked at the FB process footprint (gumbo.exe) and its 500Megs, 5 times larger than anything else including Photoshop. I was also running Flex Builder 3 at the same time. Is there some known issues...googling I found a similar complaint for Mac.
I had the help window docked with the mode set to "related topics", if I close the help window the slowness goes away. In Flex Builder 3, I always had the help window open and set to "related topics" without any noticable performance problems.
Try turning off LiveDocs (it's in the top of the Flash Builder preferences page). We've found a bug in Java with http requests not timing out when they should. It's fixed in JRE 1.6 so this only repros with JRE 1.5.
Hope that helps!