Missing album art and slow Music app

Out of the blue, some, but not all, of my album art and podcast have disappeared on my phone. They appear normally on my iMac. At the same time this problem sprouted, my Music app is extraordinarily slow when going bac form a play track to a playlist or album listing.

I tried unsyncing and resyncing the music tracks. Now, there are different tracks missing album art, but most still have no art.

Thanks,

Andy