Chrooted from LiveCD: Bad IO performace

my Ubuntu Server installation is broken, so I grabbed a Ubuntu Desktop CD, mounted my Ubuntu installtion on my hard disk and chrooted into it. But now my IO performance is really bad:

# time dd if=/dev/zero of=/tmp/foo count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 13.3886 s, 0.0 kB/s

real    0m13.390s
user    0m0.000s
sys     0m0.000s


# mount
/dev/sda4 on / type ext4 (rw,relatime,errors=remount-ro,data=ordered)
devtmpfs on /dev type devtmpfs (rw,relatime,size=4076308k,nr_inodes=1019077,mode=755)
none on /proc type proc (rw,relatime)
sysfs on /sys type sysfs (rw,relatime)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
none on /run type tmpfs (rw,nosuid,relatime,size=816372k,mode=755)

When I call hdparm -tT /dev/sda it takes ages and this is reported in my logs:

[19408.841340] INFO: task hdparm:14378 blocked for more than 120 seconds.
[19408.841345] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[19408.841348] hdparm          D ffffffff81806200     0 14378  14149 0x00000004

Does anyone have a clue what's wrong here?

Thanks!

Marta