Seeing as the iPad 2 and the iPhone 4S have the same chip, describing my experience here seems appropriate.
Pretty much no changes from 9.2.1 to 9.3 (for me anyway) at first there was a tiny bit of animation lag, but that cleared itself up. Apps open maybe a tad slower, but nothing to drastic. Battery life still sucks, but i manage. There has been a noticable delay in wake up time though. If i put it to sleep and then wake it up, the screen takes about 3-4 seconds to actually turn on. Seems im not the only one having this issue.
Other than that, 9.3 extends the life of my little 4S for just a few more short months.