4S thoughts thus far:
So far 4S performance on PB2 is pretty stable. About the same "speed" as 8.4 but with much smoother animations I am finding. Especially unlocking the phone, and swiping between screens, there is no longer any "stuttering" or "frame drops" as there noticeably was with iOS 8 on the 4S. Its not as fast as my 5S but it feels more "solid" than iOS 8. Also I've noticed the App store loads much faster than it did in iOS 8. This was a nice surprise.
Widgets load quickly, almost instantaneous (seems to be better than iOS 8 where there was a pause) - But animation to drop down notification centre is a step too laggy though and un responsive at times. Same thing with control centre. Though after swiping up and down a few times, it does improve but its still a "step" too slow
My biggest peeve with the PB2, is the keyboard is very laggy and there is a delay with typing. This is what I am finding most annoying about PB on 4s so far, as this isn't an issue with my 5s.
App launch delay is annoying, it is here as well - same thing on 5s (and other users have mentioned they've experienced it too)
Battery is better than 8.4 - so far I've managed about an extra hour of battery life than iOS 8, but I have not focused on testing that yet more so performance and app bugs. For reference my last iOS 8 battery test was just over 3 hours before I needed to recharge, whereas this about about 4 hours. I did not use power saver mode, and I was downloading and installing a ton of apps.
I ran into a bunch of bugs that I've sent off to apple via feedback.
Initial iOS 9 impressions on the 4s are much more positive than iOS 8.