yep, ambient temp is always showing on my obc too.
it's fairly quick to respond. inside my garage was 80F the other day with ambient temp outside at 74F. took the car 2-3min and about 1/2+ a mile of driving to get there. the car seems to respond faster to cold temps. like going from 50F inside my garage to 35F outside. for that i drive around the block and temps responds within 1/4 a mile.
it could be that the plastics and metals around the thermocouple is heated and takes more time to cool.