Apple App Store submission review time vs update time
Solution 1:
Updates are generally processed faster than the first submission of an app for review, but not drastically faster. It's usually around a day shorter (5 days) compared with the average 6 or 7 days for a first submission.
Note that iOS 8 has caused an increase in submissions and a consequential increase in review times; Apple may be prioritising updates more to make apps compatible with iOS 8.
Solution 2:
Check out this resource http://appreviewtimes.com/ . Very handy but it doesn't distinguish between first submissions and updates unfortunately.
Solution 3:
If you check Apple's developer site they list the percent of new and updates approved in 5 business days or less. I'm not sure how accurate this is because in my experience it usually takes 7-10 days to get either a new app or update approved and far longer if you get rejected.
Apple's review claims link: https://developer.apple.com/support/appstore/app-review/
Actual times from statistical sample: http://appreviewtimes.com/
Between the two there is a big discrepancy, or I am just really unlucky as it takes about 10 days (7 business days or so) for me to get updates through for You Doodle.