To solve the speed problem with a Intel Corporation Centrino Advanced-N 6235 AGN card in a Dell Latitude E6540 do the following in a terminal:
echo options iwlwifi 11n_disable=8 | sudo tee /etc/modprobe.d/iwlwifi.confReload the module (sudo modprobe -vr iwldvm iwlwifi && sudo modprobe -v iwlwifi) or reboot.
The speed will increase from sometimes 20Mb to over 100Mb per second, depending on your bandwidth maximum.