I've been using a combo Wifi/Bluetooth Intel Centrino 6235 mini-pci wireless adapter on my laptop with Ubuntu 12.04 for many months without problem.
But since I've upgraded to Ubuntu Trusty 14.04 LTS and I've started to use Bluetooth audio, my wifi speed has dramatically dropped.
Wireless performances have become so bad that system seems to freeze for few seconds, web surfing is sluggish and file transfer can go down to 20 kB/s !
This problem seems to come from some kernel evolutions where default parameters for iwlwifi module have changed and are not suiting Intel Centrino 6235 wireless adapters.
This article explains how to get back normal performance on a Intel Centrino 6235 mini-pci wireless adapter under Ubuntu 14.04 LTS.
It has been tested on a Ubuntu Gnome 14.04 LTS Amb64 installation but should be applicable to any other 14.04 flavor.