But this was introduced in order to equalize the chances between bots and manual curators. Otherwise, the bots would vote immediately, taking most of the curation rewards.
If someone finds a valuable post, it is usually more profitable (in terms of curation rewards) to vote much earlier (even in the 5th minute) than to wait until 30. The key is that the formula for the curation rewards takes into account also the sum of rshares before your vote so if it is low enough, you gain more from being before others than lose from voting before 30 minutes.
I pasted the link to my last post here: http://steemer.pl/curation/post-curation.html
And this is the result:
The best performance was achieved by the person who voted in 4th minute! :)