?? I have used all three of them (on same machine, different partitions) , KDE is the slowest on all account and kwin cpu usage is high. Yes, kde would be faster if you disable all effects but then why not just run LXDE? Unity in 13.04 is very fast, but can be made even faster and smoother with the compiz-experimental ppa (though one should get that sweet spot and stop upgrading as there are some risks)
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