There is a slowdown in some areas while some other areas are faster.
It won't be very noticeable but incremental slowdowns with each iOS version results in a much slower iPhone over time, as you can see with how older iPhones run the OS.
You shouldn't notice much of the slow down but it is more of a system wide thing. It should improve with future betas but will likely be still there when the final release comes out. There are videos on youtube comparing the performance I think. It's nothing to worry about at this stage but its your call.What areas are slower/faster?
Right now I'm happy with iOS 9. I don't want a slow iPhone.
I think you'll be fine updating all the way to iOS 11, but after that you will most likely see slowdown.I would like to get it when it's released but I don't my iPhone to become slow.
Apple is no angel when it comes to these things, but I'll tell you those new features look like a major upgrade. Definitely want them.