As I wrote earlier I've had some problems with the iwlwifi driver and my Intel 3945 wifi card. One workaround brought to my attention by again by Jeremy Olexa's blog post is to run G mode only.

This can easily be done in OpenWRT Kamikaze with:

ipkg install wl

In the config file /lib/wifi/broadcom.sh add the following line just after the killall statement:

wl gmode GOnly

Restart network/box.

With this workaround speed is back to normal, thanks to the initial blog post by Markus Golser here.