since you're on Win8.1, when you're on the traditional desktop view, right-click the taskbar at the bottom (a blank space) and open Task Manager, and keep it open on I think the first tab. it should show a list of programs and then 4 columns: CPU, Memory, Disk, and Network. do that before you connect your wifi to see what the base line is, how much you're using and what process is being the biggest hog. it should be all sub-20% anyways.
now when you connect wifi and do what you would normally do when online (keeping that window open and visible), see what changes and which one is sitting at the top when things bog down. one of the 4 columns should start glowing red and the biggest hog for that column shows up at the top. which column is sitting at over 80% or even over 95%? what's the top hog? post a screen-shot if you can, or just write down what it is.