It's slow. It isn't unbearable or anything but it definitely is slower than iOS 6. Animations are longer and laggier, you can disable them but in a side-by-side comparison with another 4S on iOS6 it's still slower. Apps (Safari, Messages, etc) are generally a little slower as well. Also, when waking the phone from sleep there is a slow fade-in animation, it looks nice but when you're trying to check the time quickly it gets annoying.
I'll put it this way—I was quite content with my two-year-old 4S (aside from deteriorated battery life) and was even considering squeezing another year out of it. iOS 7's performance made me change my mind, the phone had not felt particularly slow or outdated to me until I updated to it. All that said, on my new 5S it is great.