https://github.com/steemit/steem/commit/6300a795cae5e78f68b52d03ea0918bf8354e600

If you upvote a post within 30 minutes of it posting, your curation reward is penalised. If you think the post is really worthy of the early upvote, you need to weigh up the benefits of being "first to vote" against the early voting penalty