I have a ThinkPad T420, which comes with an Intel Corporation Centrino Ultimate-N 6300 wireless adapter. The card is recognised out of the box, and it connects to my home WiFi. There are two kernel modules loaded:
iwlwifi. Typically, after a minute or two the internet connection becomes so slow that it is virtually lost. I am running Slackware64 14.2 with kernel 4.4.217. It seems I finally found a solution in the Ubuntu forums.