Kio slows down system when copying large files to USB disk

[rdbert]

When copying large files (more than 500MB) to USB disk, the entire system become very unresponsive, mouse movement jitters, and the music playing in VLC stutters/choppy. The USB disk is a 8GB flash drive formatted with ext4 (NTFS also have the same problem). Tried reducing the cpu priority and io priority of kio_file processes, but condition marginally improved.

Interestingly, when copying the exact same files using 'cp' instead didn't reproduce this behaviour.

I am using Kubuntu 12.04, 64-bit, with the following architecture and kernel version:
3.13.0-37-generic #64~precise1-Ubuntu SMP Wed Sep 24 21:37:11 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

For your reference, the installed version of the KDE can be obtained from the dolphin's version:
dolphin:
  Installed: 4:4.8.5-0ubuntu0.1
  Candidate: 4:4.8.5-0ubuntu0.1
  Version table:
 *** 4:4.8.5-0ubuntu0.1 0
        500 http://mirror.unej.ac.id/ubuntu/ precise-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     4:4.8.2-0ubuntu2 0
        500 http://mirror.unej.ac.id/ubuntu/ precise/main amd64 Packages