iTunes 12.2.2 pauses with spinning wait cursor when selecting tracks in playlists

My friend is a radio DJ and has a large music library which she curates with extensive metadata edits and loads of playlists. Her iTunes Music Folder sits on an external USB 3.0 drive as her 15 inch Retina MacBook Pro is limited to 256GB of flash storage.

Since she updated to iTunes 12.2.2 she can no longer use her Mac for live broadcast at the radio station (she uses a DAC) because when she chooses a track the wait cursor spins for up to 20 seconds or more, every time. It's driving her crazy.

I'm thinking about deleting cashes and rebuilding the Spotlight index but haven't done so yet. Has anybody got a fix or some sage advice before I dive in?