High iowait when copying large files in Linux

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Always paid attention to another oddity in the disk system in Linux:
When active drives, e.g. when copying a file (no matter between different disks or not) the CPU usage grows up very much (most of it is in iowait, usually completely focused on one core) and the system subjectively begins to work more slowly, becomes less responsive.
The winchesters I have SATA2 in the BIOS before there was mode IDE to SATA, recently put AHCI, no difference noticed)
Stone is a dual-core Phenom II x2 555.
Tested hdparm'ω the speed of linear reading — to a new terabaytnye 100 MB/s, for the old screw on 320Gb — 70 MB/s.
I don't know how these values are normal.
A large file copy from the old screw on new — about 50 MB/s.
System is ArchLinux x64.

I would like to hear the opinions of those better versed in working with Linux disks.
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12309 :-(
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This is normal. Do not worry.
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If you copy something big from system or for system screw or the screw(s) where is your $HOME, then logically the system will be subjectively slower.
Big iowait — is also a logical thing, it is usually the higher, the more extent-s (read: the more fragmented) files.
I have when copying files on a separate screw, significantly system becomes slower at all.
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