Comment 2 for bug 1410260

Qt's documentation says this:

To force asynchronous image loading, even for image sources that do not have the asynchronous property set to true, you may pass the QQuickImageProvider::ForceAsynchronousImageLoading flag to the image provider constructor. This ensures that all image requests for the provider are handled in a separate thread.

This is exactly what thumbnailer does (thumbnailgenerator.cpp:29). This should make it fully asynchronous. All threading related to this is done by Qt internals and they are not exposed to QQuickImageProviders. This would indicate that the blocking happens somewhere inside Qt and thus is not caused by thumbnailer.