There seems to be an issue with the new Intel Centrino Advanced-N 6235 wifi chip when running in N mode. This problem occurs in all current kernels, as of this writing  <= 3.7.

The problem lies in the iwlwifi driver and disabling the N mode support seems to be an reasonable solution for the time beeing.

First check if you get an improvement:

$ sudo modprobe iwlwifi 11n_disable=1

If it performs better make it permanent:

$ sudo su#  echo options iwlwifi 11n_disable=1 | sudo tee /etc/modprobe.d/51-disable-6235-11n.conf