how to get an sfml 2.2 window to constantly show fps

I'm looking for a way to accurately show the fps of my pong game (that i'm going to change into a more involved game later), without loss of the actual fps while doing so. The source code of my program is here : http://ift.tt/1R42o5c .

I'm also looking for a way to optimize it as there is a definate performance problem somewhere... Thank you!