Talking through my hat, but actual "boot time", i.e. time for the firmware to be running in steady state, should be less than a second.
Even if the module isn't associated with a network, it should only take a very few seconds after power up (just a SWAG judging from experience, I haven't actually measured it, but 3-4 sec has to be enough, nowhere near 15.)
My experience is with the ZigBee firmware, not sure if ZNET would be different or by how much.
Not as an answer to the issue here, but the product manual does specify that coordinators and routers should be mains-powered. I'd probably try an end device and pin sleep, rather than pulling the rug out by switching the power.