Speedo, I'd suggest that when compiling your music you keep it split into lots of directories with about 20 tracks, or an album, in each.
When the Zumo starts up it scans the music to build its list. For some reason if all the mp3s are in a single directory it takes more 'processor power' and the unit then doesn't draw the map or navigate. When I had 630 tracks in a single directory it took over 2 mins before it would draw the road and navigate. Split them into multiple directories with 20 tracks in each and it now takes about 30 seconds.
Also reduce the quality to 128bit. You'll not hear the difference on the bike, but will save space.