Kio slows down system when copying large files to USB disk

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