The page https://doc-snapshots.qt.io/qt6-dev/build-optimized-qt.html needs a few corrections.

The advice to use

set_target_properties(
        ExampleApp
    PROPERTIES
        QT_QMLCACHEGEN_DIRECT_CALLS ON
        QT_QMLCACHEGEN_ARGUMENTS "--only-bytecode"
)

is not correct. QT_QMLCACHEGEN_DIRECT_CALLS is available with the commercial-only Qt Quick Compiler Extensions only. Users who do not use them should also be told what their options are.

"QT_QMLCACHEGEN_ARGUMENTS "--only-bytecode"" turns off the compilation to C++ in the Qt Quick Compiler and so brings any C++ related optimations out of order.

See https://doc.qt.io/qt-6/qt-add-qml-module.html for more information