My girls loved tummy time and could lie there for ages - they didn't crawl until 9 and 10 months respectively so liking tummy time isn't an indication of early crawling. Both were also not walking until after their first birthdays (14 and 13 months) and they both have well developed language. I think kids who move later are too busy developing their other skills like talking so they tend to be better talkers - but that's just my anecdotal evidence.