There is a penalty for voting earlier on a post. This is not a negative penalty. It is a penalty in a form of a reward or bonus for the author of the post. If you vote early, what you're doing is valueing the authors post giving all or some of your curation rewards based on when you vote. After 30 minutes, 100% of the curation rewards go to you the curator of curating and 0 to the author. Anything less than 30 minutes is split between the author and the curator in terms of curation rewards.

Details:
Voting above 30 minutes into the post, the author gets 0% whilst the curator gets 100%.
Voting above 20 minutes into the post, the author gets 54% whilst the curator gets 44%.
Voting above 10 minutes into the post, the author gets 89% whilst the curator gets 11%.
Voting above 5 minutes into the post, the author gets 97.2% whilst the curator gets 2.8%.
Voting above 3 minutes into the post, the author gets 99% whilst the curator gets 1%.

Hope this helps :)