Username or EmailPassword
My take on this issue is that while KDE4 is somewhat heavier than other environments, the difference in CPU usage or memory consumption does not matter much on most of the hardware people use.
The problem with KDE is that it puts strain on the underlying subsystems and hardware drivers. From my experience, it works fairly well on legacy hardware, where most of the new stuff is disabled anyway, or it requires top-notch drivers supporting all the new and fancy features. The ugly truth is that most of the linux hardware drivers are half baked and require application writers to carefully maneuver their way through supported features.
On my system KDE4.7 is constantly using 100% of one (of four) CPU cores. This of course makes the whole system run like crap but I doubt this is how it is supposed to work (I mean, no one could screw the implementation up so badly, even if he/she tried very hard). More likely, there is some conflict between KDE and some of the subsystems (audio in my case).
Ultimately, though, it is the job of KDE4 programmers and distributors to make their product work well. The users have all rights to complain if it is unusable out of the box and it is not their job to fix it.