I have had huge lag issues while doing i/o operations(sometimes the desktop would come to a halt when copying files to a usb pendrive) until i upgraded my archbox to kernel 2.6.36. If u can try updating your kernel.

I'll be moving to Kubuntu 10.10 soon and see if anything's different . In the meantime I'll check to see if my kernel does memory swapping too often .

As for the over-swapping suggestion:
I was doing a 3MB/s download with uTorrent on a separate partition and was experiencing the plasma unresponsiveness. I decided to turn off swap with the swapoff -a command and to see if there's any difference. No, Plasma still had hiccups. Interestingly I decided to turn on compositing (Xrender, because works faster for me for some reason), and voala the desktop experience just got smoother .

So why is Plasma without compositing much more unresponsive, than with it turned on?

PS. I'm updating to a newer version of Kubuntu soon and will see if there's any difference there, but I doubt it, since I was experiencing this issue ever since KDE 4.2 (or maybe KDE 4, but I started using KDE after 4.2).

Very interesting, this indicates possible issues with Plasma in other areas. With swap disabled and compositing also disabled, can you please monitor the disk activity of Plasma using iotop/strace and the CPU usage of Plasma?

Well, I ran some tests and the interesting thing was that when copying large files, I had 100% CPU usage (kio_file), but I was told it's ok. Plasma's CPU usage weren't high (only 1-2%), but was having regular requests to the HDD (I'm not sure if it's necessary) - tomorrow I can show it in a video.

Plasma's regular requests to the HD are the likely cause of these responsiveness issues. Please use a program such as strace to determine which file(s) are being altered, then file a bug report at bugs.kde.org containing that information.

Thank you for your advice. I'll read strace's manual and try to determine wich files are being accessed .

I am experiencing the same problem.
Arch Linux, KDE 4.8.1, BTRFS

Intel Core duo 2x2GHz and 2GB of RAM.

Have you been able to determine which files Plasma is accessing that cause this to occur?

I'm experiencing the same problem too.
Gentoo Linux AMD64, KDE 4.9.1, EXT4
It happen during building anything (-j3) in chroot after 2-3 hours. Also I noticed often starting few kio- processes in another time.

My machine:
Intel DualCore 2.93GHz and 4GB of RAM.
I will try to to determine which files Plasma is accessing that cause when it appeared again.

Just a note - lsof and strace may be helpful in determining which files are being interacted with by Plasma Desktop. The results of your investigation will be quite interesting.