Without offending GT, I can say that WebGUI is a far more complex product than Gcommunity and Gforum... It contains more than 100,000 lines of code. So you can't compare GT and WebGUI.
Just browse some WebGUI websites and you'll see that it can be very fast if well configured (mod_perl).
From the official F.A.Q.
http://www.plainblack.com/webgui/faq WebGUI is fast on your server, but it's slow on mine. Why?
WebGUI is a complex application. It takes a few tricks to make it run well. You can find a whole chapter on performance tuning in Ruling WebGUI. But to get you started, the best thing you can do is install mod_perl.
Max
The one with Mac OS X Server 10.4 :)