QML significantly slower when build with newer Xcode

Hello,

I recently upgraded, my MacBook, Xcode and Qt, to be up to date with all development tools.

Right from the start I noticed a significant delay, or lag, when navigating one of our applications.

I'm comparing here an app currently live in the AppStore with a freshly compiled app locally, both with the exact same source code.

My investigation lead me to the following conclusion:

The issue is with the Xcode - and therefore iOS SDK - version that Qt(the framework itself) is build with/against.

I tested with a minimal example, attached to the report. The 2nd page of the StackView has a few creation heavy components, that should always take a measurable amount of time to create/load in. Especially the FileDialog is the heavy hitter here. 

I downloaded, via the online installer, Qt for iOS 5.15.2 and the source code. I also downgraded Xcode to version 11.5, a Version from the middle of last year, via this side: https://developer.apple.com/download/more/ 

And build Qt for iOS with Xcode 11.5 and that source code from the installer.

I used an iPhoneX (my daily driver) as the hardware to run these apps on.

• When the example app is build with Qt 5.15.2 (precompiled libs, provided by the online installer) the 2nd page takes about 900 - 950 ms to create and display

• When the example app is build with Qt 5.15.2 (compiled with Xcode 11.5) the 2nd page takes about 200-220 ms to create and display

Nearly 5 times faster, that can and does make the difference between a fun, fast and snappy application and doubting yourself (Did I press that button or not?)

There seems to be something that is turned off.

Greetings