I play RS3 in windowed, because the clients fullscreen doesn't allow for overlays. I went from ~40 fps, to ~13 fps with the fps counter turned on. As soon as I turn it off, my fps goes back up to normal.
I tested it on World of Warcraft: Cataclysm, where the fps counter does remain upright. However, as soon as I switch to windowed or windowed borderless, my fps drops from ~175 to about ~144. Now, in this case that drop wouldn't matter that much, because the base fps is already really high. But in the case of RS3, it really affects gameplay significantly. Obviously, I have turned off the fps counter. Already have one built in another tool for RS3, so don't need to keep an eye on it through Cortex.
My rig:
