I can't recall which article I read, but it said you'll get a performance hit close to, but not quite as bad as iOS 8. My iPad 3 suffered greatly on iOS 8, but is slight less annoying with iOS 9.1 beta. Also, the small screen on the 4s forced you to scroll more than you did before, compared to phones with bigger screens.
The article went on to say that they thought it was still worth upgrading due to the added features you get and that developers aren't supporting ios7 much now. Occasionally you will need the later iOS versions to even downloaded some apps.
Someone will correct me if I'm wrong, but I think you could download the iOS 9.1 public beta and undo it if you don't like it. However, I'm not sure it you'll get iOS 7 back vs iOS 9.0 when you undo the beta. Someone more knowledgeable will have to chime in.