When I copy files the CPU usage is about 100% in my AMD Athlon II X2 260 (max speed 3200) dual core processor.
System version KDE 4.7.4 on Kubuntu 11.10. The same problem is in KDE 4.8 on Kubuntu(live usb) 12.04 beta2.
There is no matter which program to use Dolphin or Konqueror or cp(konsole) and no matter which filesystems (ntfs, fat, ext4) and no matter from HDD(ext4) to USB(fat32), or from USB(fat32) to HDD(ext4), or from HDD(ext4, ntfs) to HDD(ntfs, ext4).
The same systems (Ubuntu 12.04 beta2 and Ubuntu 11.10) when copying files use max 20% of CPU, so I think this problem refer to KDE.
Sorry, if my english not so good, but I hope you can understand .

When I use top(in konsole) to see what happens it show

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Cpu(s):  7.0%us,  3.5%sy,  0.0%ni,  7.0%id, 82.2%wa,  0.0%hi,  0.3%si,  0.0%st
...


when copying and

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Cpu(s):  3.7%us,  1.8%sy,  0.0%ni, 93.7%id,  0.5%wa,  0.0%hi,  0.3%si,  0.0%st
...


when not copying files.
man -k top:

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...
wa  --  iowait
          Amount of time the CPU has been waiting for I/O to complete.
...

top(in konsole) in Ubuntu 11.10 and Ubuntu12.04 shows about 100% CPU usage(about 75% - wa), but Gnome System Monitor shows only about 20%.
So, I think this is not a prolem