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No, until now I thought votes would become less value before the 5 minutes.

True, if nobody voted before you do, then it would be best to upvote after exactly five minutes, but that's not the case if some big upvotes come in earlier ...

So the optimum is to upvote close to the five minutes limit but still before any big upvote has been given. The reason is to make the decision when to vote more speculative, so that bots wouldn't always 'win'.
However, it hasn't changed the fact that manual curation loses.

Ok. Now I understand. Could this be also a mechanism steembasicincome can profit from?

Everybody can try to profit, by maximizing the date of his upvotes ...