This is a great question!
Please watch Paul's recent video Hiding The Keyboard.
In this video Paul discusses a problem with hiding a numeric keyboard after entering a dollar amount.
This was solvable with older versions of iOS. But, indeed, it was a pain.
With recent updates to iOS, the Swift language received a new state variable named @FocusState. This gracefully solves the keyboard problem in a very Swifty (declarative!) way.
So, if a developer says they are optimizing for iOS 15, one thing they might be doing is stripping their application of the previous keyboard hiding hacks, and implementing a more elegant @FocusState based solution.
In this case the optimization may not make the application any faster. (This is a common understanding of optimization.) But it will certainly make the application easier for the developer to maintain and improve.