What should I do about this?
Don't use sf::Clock, it's a heavy class that can only be used on computers that support OpenGL 3.1.
Seriously, of course it's not slow, it simply reads the elapsed time. So the error is in your own code, and we need to see it.
And next time, please post in the correct forum. I'm really tired of lazy people who post everything in "General discussions", whereas there's a "Help" forum dedicated to each SFML module