Poor (ata-?)performance with VIA-based chipsets

Hi,

I have a really bad performance with my FreeBSD system for years. After
long discussions[1] at the german BSD-community we figured out that it
seems to be a VIA-chipset based problem.


# dd bs=1m < /dev/random > /dev/null
does not slow down the system

but with

# dd bs=1m < /dev/zero > tmpfile
the system becomes nearly unusable after some seconds. If I start
mplayer to watch TV with my bt848-card the desktop environment freezes
for round about 10 seconds before switching a windows or something like
this.

Any ideas how to fix this?

[1] = http://www.bsdforen.de/showthread.php?t=19426&highlight=performance
(german language)