This is the time it takes for news in major subreddits to gather enough votes on the "new" tab to make it to the main subreddit front page, then the actual front page.

Smaller subreddits seem to be less affected by this, which is why /r/toosoon (a subreddit dedicated to dark humor related to current events) is often surfaces news hours before other subs for people who have it in their subreddit list.