So I tried to test your experience with intel and amd systems. I own a Athlon64 3700+, used it until end of oktober. It had a nvidia 6600GT inside. Since then i have a i7 2600K (running the iGPU).
Another PC in the room runs on an AthlonII X3 435 (3x2.9GHz) and a nvidia 220 GT.
All PCs running kde4 (3700+: kde-4.6.5, the others kde-4.7.3)
Running 5 Firefox-windows, some with dolphin, many OOo-windows made the UI quite laggy in the X3, my 3700+ even cried when running kile + one OOo + one firefox with 10 tabs.
I never experienced any lag on my i7, so i opened firefox with 15 tabs, running several websites, including Flash videos. Opened 30 more FF-Windows, tvbrowser (java), kile and kdevelop.
At this point, Desktop-Effects from kde4 were not that quick anymore - Alt-Tab needed a fraction of a second to show me the cover switch, but then, the windows switched smoothly.
So, ok, I need to do something stressing the CPU...
building kdelibs with -j10, building gentoo-sources with -j5, importing kdelibs into kdevelop, in parallel do edit projects with large files in kdevelop and kile (ok - I can't edit two files at the same time, so I switched forth and back).
All 8 Cores pushed to 100%, and - filnally - editing became some sort of laggy! The Cursor needed a fraction of a second to jump when typing!
But - hey - everyone knows kwin is not that performant (remember: Opening some windows on an AthlonII X3 + nvidia GPU and doing nothing stressful but editing some files in OOo made the UI laggy), so I disabled desktop effects, et voila, lags disappeared! Editing goes as smooth as everytime (still, kdevelop importing kdeibs, 15 threads compiling kdelibs+kernel on 8 cores, java running, many firefox windows/tabs). To give a minimum load on the GPU, I started the only game I own - xmoto Loading a replay, arranging windows so that xmoto is not covered (note: You must bring the cursor above the xmoto-window in order to start playback of the replay)- anything runs fine, no lag when typing in kile, kdevelop autocompletion comes down instantly, even xmoto runs smooth as if there was nothing!
So I really can't understand how you could experience such lags with an intel-based system, even when using the iGPU).
What kind of GPU do you use in your Bulldozer-systems?
If you experience Lags on intel-systems when running the iGPU, just disable desktop effects (Or use a more performant window manager).