Copying disks using cat makes 2.4.20 very unresponsive

Hi,

If I copy disk or partition eg. /dev/hda1 to /dev/hdc1:
cat /dev/hda1 > /dev/hdc1 the system becomes __highly__ unresponsive The same is for:
cat /dev/zero > /dev/hda1
Processor is about 90 % idle
'cat' takes about 3-5 % CPU.
Load average is about 5
Working with X is impossible (about 1 second freeze every 1-2 seconds). renice -19 for X makes no difference.

What is really strange, if I use 'dd' eg.:
dd if=/dev/hda1 of=/dev/hdc1 bs=1024 count=1000000
there are NO freezes at all.
The system works smoothly. Load is about 1.2.
dd takes for some time 30% CPU and then until finishes about 0.2 % CPU. I have tried bs=512,1024,2048,4096,8192,16384.
It works smoothly with all above values.
Also cat /dev/hda1 > /dev/null doesn't hurt.

My system description:
I have linux 2.4.20.
System is based on RedHat 8.0. Tried with RH 9.0 also. Motherboard ASUS A7V266A (via KT266A chipset) 512MB RAM
2 IBM SCSI disks connected to Adaptec 2940U2W CD-RW, DVD-RAM connected to the same Adaptec card 2 IBM IDE disks connected to motherboard controller. IDE disks use DMA, and are set to UDMA-100

System is installed on SCSI disks.

Please CC me.

Regards,

Olaf Fraczyk