Putting on O2 masks immediately is so second-nature that is likely not it. We do it all the time, in normal operations, and for a rapid decompression it is the first thing, every time. It takes 2 seconds with modern inflating masks.
You do not start the descent first, you put on O2 first. Always.
I would agree—but what if the leak happened in the cockpit area? That means the loss of cabin pressure at the worst place possible—and the flight crew may have been incapacitated even faster than normal.