I’ve got a nice shiny Samsung Series 7 laptop with Broadcom based 802.11n wifi in it. I’m using xubuntu on it and it was working very well, but every so often I noticed that the wireless connection was running very slowly.
Eventually I realised it was only happening when it was running on battery.
I delved into the power settings and found that the wireless card was being put into a power saving mode. After a lot of searching I found out how to turn this off.
iwconfig wlan0 power off
Once I have run this command the speed returns to normal, and there is only a tiny increase in power consumption.
To make this permanent you can make a file called /etc/pm/power.d/wireless containing the following:
/sbin/iwconfig wlan0 power off
This will be run automatically by the power management in Ubuntu.