*sigh* Look, I'm annoyed as anyone when people complain about things not running smoothly in betas, but honestly. 5 dev betas and 3 public betas later, and iOS 9 does not run well on an A5 device. I may be completely wrong, but I highly doubt it will magically stop dropping frames with every animation before the GM. Trust me, I want to be wrong. I just don't see it happening.
I think there are variable experiences for users. For example, I found the beta 1 (dev beta 1, not PB1) to be absolutely unusable on the 5s. It was beyond laggy and terrible animation speed. Very unresponsive. Reminded me of the iPhone 4 on the original iOS 7 installations lol.
My current experience on the PB3 on my 5s is great, its the smoothest its been in a long time easily the smoothest on iOS 9 betas. I have no lag, no dropped frames, multitasking is very responsive and fast, and my notification centre and control centre appear very responsive.
However you can read through this thread that others on a 5s have not had the same experience, in fact theirs has been disappointing at best, reporting laggy frames and stuttering or non-responsiveness
Myself and another user on here found 4s to be very solid on PB3. Its the best my 4s has run since 7.1.2, which I had on the device up until only a month or so ago. I've had various iOS 8 versions on another 4s, and experienced 8.4 on my 4s which now has PB3 on it, but my experience overall with iOS 8 on the 4s has been abysmal. Clean installs, hard resets, a lot of time spent on various techniques and such....didn't fix anything, its been a terrible experience for me.
However, a good friend of mine loves his 4s on 8.4. He's had a solid experience with iOS 8 from 8.1 onwards. Today he put the PB3 on and he's been texting me nothing but bad stuff all day haha. I've seen his iPhone with iOS 8 at various times throughout the past year and it was a smoother than my experience with it. His battery drainage, how stuttery its been for him, etc... has been the opposite of my 4s iOS 9 beta experience.
Point is there will be people who experience both sides. I am not sure why exactly, but what is a smooth experience for me can be dismal for another user on the same device, and vice versa.
I wouldn't be so quick to write it off until iOS 9 is actually released, But its unfortunate you're having the problems you are, and understandable why you're frustrated....hopefully it improves by the time the GM comes out.