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The toolkits apparently make very bad use of the protocol, but the biggest culprit is Xlib. XCB supposedly improves network performance considerably, but no toolkits use it as of yet.
Xlib only uses XCB essentially as a transport layer. It still has the same bad implementation on top of that.
I think you need to check your driver configuration or talk to some X.org devs because unless you have a really crappy gfx card (and it sounds like you don't), you shouldn't be seeing those issues. I have a T500 with a Radeon 3650 in it and it is quite snappy and I don't see artifacts or tearing. Compositing is also quite usable, which will significantly reduce flickering (just like in Windows Vista/7).