If you have problems connecting to networks in general or your link quality is very poor, try to disable 802.11n, and perhaps also enable software encryption:
create system modules configuration file: /etc/modprobe.d/iwlwifi.conf
adding this as options for the wifi device module: options iwlwifi 11n_disable=1 swcrypto=1
Your post or your actual issue is fixed?
Network slowiness on wifi can be caused by interference loacally too… or some update of the kernel and not rebooted e.t.c.