Also check that you have 300MB free space in /tmp.
I just did the same test on my p630 workstation ...
time dd if=/dev/zero of=/junk/dat bs=1024 count=30000>
300000+0 records in
300000+0 records out
real 0m4.79s
user 0m1.00s
sys 0m6.21s

Also note the AIX likes 4KB disk writes so I tried
time dd if=/dev/zero of=/junk/dat bs=4k count=75000
75000+0 records in
75000+0 records out
real 0m2.33s
user 0m0.29s
sys 0m2.97s

Also, remember that this write test is not actually doing disk I/O - the
data is put in the filesystem cache and written later. So kernel tuning on
the filesystem cache options and the amount of memory can effect the
performance more than the disks involved!!

Hope this helps, Nigel nag@uk.ibm.com