sustained sequential disk IO runs interactivity into the ground

[eng at donut.ugcs.caltech.edu]

I was doing a dd of dev/zero into a file on a UFS2 filesystem
(softupdates disabled) on a clean 5.4-R system.

an exec of top took approximately 30 seconds to complete
and then revealed sys: 13% and idle: 87%

quit, repeat five times, same delay, same numbers each time.

FreeBSD 4.11 managed this with only a minor interactivity
hit.

What's going on here?  Any ideas?