Xbuntu / AMD
Short description of error
Checking "Active Editor Only" used to only play in the editor the mouse cursor was hovering over (which would drastically help with the framerate), however now it just plays in all the editors regardless of where the mouse cursor is.
Exact steps for others to reproduce the error
Go to the "Playback menu"
Uncheck "All 3D View Editors"
Uncheck "Animation Editors"
Check "Active Editor Only"
Split the viewport so you have at least two of them open
Move the cursor over to one of them
...it plays in all editors...