Avnx EO,
I do not know all the compass systems, but the GM9 in our Lynx helicopter has a socalled "fast slave" button.
Slaving the compass with this fast slave mode uses a speed of 30 deg/sec so it takes maximum 6 seconds...
Indeed if you have to wait for the slaving in the normal compass mode it would take much longer (i do not remember by heart, but i think it was about 1 deg/sec).