I upgrade my 6+ from 9.3.3JB to 10.0.1GM, TBH, can't feel any significant performance drop. The smoothness is more or less the same. May be you can feel some 0.1-0.2s delay occasionally, but that also happen in iOS 9 anyway.
After the upgrade, I quite like the iOS design. It's more complicated now, but that's because more function available.
All directions of the screen is utilised.
Pull down is the notification centre.
Pull right (assume is the 1st page) get you to the widgets.
Pull up have control centre (I like the Home kit short cut, that's very very useful for me)
Pull left go to next page (of course)
Pull down (from the screen centre) give you the search function.
One screen, 4 directions, 5 functions, very good utilisation.
For Apple Watch user. IMO, iOS 10 is a must, because of the Watch OS3.
And quite a lots of the stock apps has improved functions as well. So, it's worth the upgrade. For non jailbreak user, all you need is just a full backup, then upgrade. If don't like it, downgrade back to 9.3.5 and then recovery from backup. You can still do it in these few days (before Apple stop signing 9.3.5 via iTunes). So, worth a try, and nothing lose.