Code review comment for lp:~jamesh/thumbnailer/landing-20150622

My guess is that your app was running with the new QML plugin but talking to the old thumbnailer-service. There were some changes to the API as part of the rewrite (to support requesting thumbnails of arbitrary size rather than the fixed set of sizes), so the placeholder images would be due to the broken D-Bus method calls. Rebooting would have made sure you were on the new code.

One of the main causes of the old serialised behaviour was due to the old QQuickImageProvider API, which requested thumbnails one by one on a single worker thread. With this branch, we're using the new QQuickAsyncImageProvider API so we can start multiple thumbnail jobs in parallel.