The lanency in the app HDS iSound immediateley increases from 5ms to 23ms when another app only thinks about playing sound ;-)
So the latency in HDS iSound increases when SongBook+ is started.
But you can forbid SongBook+ to think about playing sound with a few changes in settings:
1) In SongBook+ settings select "Media player", then disable the 5 switches "Allow playback of sound file 1-5".
2) In SongBook+ settings select "Metronome", then disable the switch "Enable acoustic metronome".
With these settings in SongBook+ the latency in HDS iSound should go back to 5ms.