Apple times the new iOS release to coincide with their next iPhone release (usually slightly prior). Therefore many users upgrade to the latest iOS at pretty much precisely the time of the new release.

It doesn't mean that they deliberately slow down iPhones. New software is usually more bloated. Apple often does improve performance in the minor releases that follow, however.

Leaving aside the fact that Android users barely get updates, they don't usually coincide with new releases. I'd like to see how slowness questions coincide with Android software updates for corresponding phones.