The acidity of the must, the concentration of sugar, the availability
of nutrient salts, the saturation of dissolved oxygen and the
temperature of the must all seem to play a part in how quickly the
yeast spreads throughout a must and starts fermentation.

Presumably the levels of inhibitors of yeast metabolism and
reproduction and other antagonistic organisms also play a part -
though I have no idea what the effect and nature of these might be.

What else affects the time it takes a must to get going after
pitching?

I have become particularly interested in this after having one recent
5 gallon pitch which was fairly vigorous after 3 hours, but mostly
finding up to 3 days or so is a common induction time.

Many thanks, Jim