A Google Reader Help thread shows Roger giving two excuses for a slow Google Reader update. They include:
(1) A site with "few subscribers."
(2) A site using Google's FeedBurner.
The "few subscribers" excuse means that Google rather spend their resources crawling larger subscribed feeds more often than feeds with fewer subscribers. That makes a 100% sense. But site's that use Google's FeedBurner, a feed management tool, owned by Google, which so many sites use, would slow down Google Reader? Technically it makes sense, but practically, it sounds silly.
Let me quote Roger:
Sometimes Reader doesn't crawl blogs with few subscribers right away - if you only have a few subscribers, this could be the case. If you have more subscribers, are you by chance using Feedburner? Sometimes Feedburner doesn't update with your original feed's content right away, and thus Reader cannot detect new content until it is updated in Feedburner.
Most sites I subscribe to are visibile in Google Reader within minutes. Some takes a couple hours, but most are within minutes - including this one.
Forum discussion at Google Reader Help.