I can't say I've noticed anything like that on any of my machines running KDE4 & Plasma. I presume you've checked running processes with a monitoring tool like top or htop?
Yup, as I practically live in the cli I am constantly monitoring actually even with KDE4 widgets I am checking just about everything too.
I have no idea if this is because of the CPU or perhaps a kernel version which isn't agreeing to my hardware but I had to install Gnome just to cool my laptop off - I mean 200 degrees F (~100 degrees C) is hot!
Not sure what to do about this as KDE is pretty cool and has come along way since the past of which I was never fond of being a Gnome/CLI person anyway KDE3 was just icky.....
Something is definitely hinky with this though, perhaps I should post on kde.org and see what the guys say there??? Kind of as they're at the source of the issue....
Also soon I will be able to try out KDE4 on an open source derivitive of Sun Solaris so I'll see again if it's just a KUbuntu 9.04 bug or if something more major is up.
For now though I am a bit stuck with Gnome2 which is fine as I've just spent the whole afternoon and evening hacking it out to exactly the way I want it!! :-) Lots of deep configuration edits in its guts.... in fact now it looks like a cross between an old 80s Amstrad and Sun's SXCE operating system :-P
Oh well was worth a shot posting here! Thanks for the response btw :-) much appreciated, though it doesn't solve my issue.