If you download a IPSW file, it's not from Apple - it's from a jailbreaking process that's not authentic. Just so you know...
iOS 9
This is the release version of iOS 9 for iPad, iPhone, and iPod touch. Devices updated to iOS 9 can not be restored to earlier versions of iOS.
You can download the 8.4.1 IPSW straight from their servers, and it is still being signed by Apple themselves. You can downgrade quite easily by putting your phone into DFU mode and restoring it manually. This is not for jailbreaking, just an official downgrade. Of course it won't work as soon as they stop signing it. But for now it works.
Yes there are custom IPSWs out there not from Apple that are for jailbroken devices, but not for 8.4.1. I used to use all sorts of them during the iOS 3/4 days though. Many added features that weren't available on older devices, or optimized them to run smoother.
I think I'll stick with iOS 9.1 for now and hope it improves, but may have to do a full restore at some point.
My 6+ has been dropping frames since iOS 8 and 9 isn't any better. I sold my 4s a while back, but on the initial iOS 9 Beta 1 it was a good bit smoother than iOS 8 was. Nothing like 5/6/7, but better. It was a fresh DFU restore. The multitasking interface was faster and smoother than my 6+. But then it slowed down after Beta 2 for whatever reason.
Note the smoothness on B1:
https://instagram.com/p/3rz04rycV6/