This really isn't recommended any more with the modern IOS. That being said, in this case, given the sudden slow down, there may be an app that is misbehaving. I would follow this suggestion and close all the apps that have been running AND THEN perform the HARD RESET suggested above (a hard reset does not close out all programs that have been running.)
Here's an interesting article that explains more.
Stop force closing apps on your iPhone, it's not making it run faster or last longer