Originally Posted by Sazar
The speed is not intended.
It has varied wildly from when I first bought it (abysmal) through a few preview/final OS's (fluid) through some other preview/final OS's (abysmal). It is running decently at the moment on 9.3.3 but let's not get ahead of ourselves and say it works as intended. It does not. The animations speed up and slow down depending on which OS is installed, and randomly sometimes even on an OS where things are otherwise fluid.
I saw this with my 6 plus but haven't seen it yet on my 6s plus. Have you tried doing a backup and restore?