MSE and Malwarebytes are a good combination, and will not normally conflict.
It sounds from your description that the problem appeared suddenly rather than gradually. This, plus the fact that pages load slowly, suggests an internet rather than computer problem. Does the computer respond normally when offline - say, using a word processor or database, viewing photos etc?
As a first step, what results do you get if you try an internet speed test at
http://www.speedtest.net/
Wait for the map to appear then click 'BEGIN TEST'.