For example I was having slow speed with Intel Centrino Advanced-N 6205 (you can see adapter model with lspci|grep -i net). My speed went from 5MiB/s to 24MiB/s so 5X speedup!
Try this:
sudo mkdir /etc/modprobe.d
sudo cp /usr/lib64/modprobe.d/20-iwlwifi.conf /etc/modprobe.d/
add a new line to the config file as sudo: options iwlwifi 11n_disable=8
sudo gedit /etc/modprobe.d/20-iwlwifi.conf
options iwlwifi 11n_disable=8
and save & reboot