I wasn't aware this problem was so widespread. I've only read one article [on a apple fanboi site FWIW] that says it kicks in when available storage <3G (I've got >15G). I figured it was due to having only 1G of RAM which a browser can max out PDQ. Supposedly there's an app that can 'zero out' the memory which helps for a few months. Must look into that.

Forgot to add there's a definite hardware issue because whenever it slows down the back consistently gets really hot in an area near the lower center right in landscape mode (battery?).

Just installed and ran this (picked at random from several similar), and it did seem to give an immediate boost. We'll see how long that lasts, and whether it's repeatable.

Given mine a full wipe and put a fresh 7.1.1 ROM on it. Smooth so far, but I know the slow downs can come with time.

The ram cleaner is working pretty good. When an app is running slow, I quit it, run ram cleaner & relaunch, then usually it runs 'normally'.