I had some issues with a relatively slow iPhone 6 recently - it didn't register as battery throttling. So I made an iTunes backup, factory reset the device, and

I had some issues with a relatively slow iPhone 6 recently - it didn't register as battery throttling. So I made an iTunes backup, factory reset the device, and restored from backup. That got it back to about the speed I'd expect it to be. I still suspect that I'll take it in for a battery swap before the end of the year - but it might be worth trying if it's the slowness is a hindrance for you.