If it comes to preloading... Why is artwork not preloaded for the upcoming entry in advance? I have noticed considerably high latency (~1-2s) in presenting thumbnails on playback control embedded in the notification (so-called Now Playing) on queue shifting forward. The issue affects not only the Application Music Player but also the Music app.
I have tried to tweak it by firing a request for artwork, but it seems that cached responses for maximum artwork size are not reused by the Music app.