qt: images are loaded slower with LLVM

How to reproduce:

Download nightly GCC and LLVM builds for Windows, add multiple items (that have cover art) at once to the playlist. With LLVM, it takes so long (more than 200ms) that the placeholder image gets loaded for the trailing items until they are fully loaded.

With GCC, it is loaded fast enough that it is not noticeable, and it is below the 200ms threshold so the placeholder image is not loaded.

I'm not sure what is the cause of this. I have checked qquickpixmapcache.cpp (https://github.com/qt/qtdeclarative/blob/dev/src/quick/util/qquickpixmapcache.cpp) but have not found conditional code for LLVM.

We use asynchronous: true for QQuickImage (https://doc.qt.io/qt-6/qml-qtquick-image.html#asynchronous-prop) so the images should load asynchronously but sequentially within a dedicated thread started by Qt. Maybe it is about threading?

CC @mstorsjo