Nova uses a "22.2V, 4450 mAh" battery (100Wh-ish if my calculations are correct) and they get 2-4 hours of battery time. By a guestimate, the board mentioned probably consumes about 35W peak. That's a BIG chunk of their operating times, battery weight, etc. being used to just run the electronics there - no wonder a decent optimised library saves them a lot of power. And it appears to sit idle a lot of the time (or else the sub wouldn't have any power to do anything else!).
I think, therefore that the power consumption was a pretty damn big deal in this case, and there was a good argument for sorting out the processing.