If they coded it using Swift, they'd have to code and maintain a second version.
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Apple has made it clear they're doing the bare minimum to support 32 bit from here on out.
The 5S is 64-bit as far as I know.
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I am not surprised.It would take a really bad eyesight to NOT notice the huge stutter iOS 9 has compared to 8.4.1. 9.3 does bridge the gap somewhat but it will be time to buy the S7/7 by the time iOS 9 finally (if) reaches 8.4.1 levels of performance
For my wife though, it wasn't the stutter. She just felt that the device slowed down when 9.2 was applied. It took longer for things to launch and run. I haven't shown her any stutter / chop and want to see if she picks it up enough to care.