You are right; I didn’t factor in RPi boot time to that figure. I’d guess maybe 60-90 seconds total? By the time I’ve turned the ignition, backed out of the driveway, and put it in drive I have a GPS lock.
Edit - much much faster (4 secs), actually: http://himeshp.blogspot.com/2018/08/fast-boot-with-raspberry...
Something about engine oil and lubrications. Probably doesn’t harm anything in the short term but if you have time, probably not a bad idea.