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It's meant to slow bots down and reward authors. The window ensures authors capture some rewards from instant bot votes, while giving human curators time to actually read content. A bot waiting 5 minutes loses first-mover advantage to other curators who've already voted — curation rewards are split based on vote order and timing, so waiting isn't free.

But the reverse auction sends the rewards back to the reward pool and not directly to the author. But what is the downside of bots waiting exactly 5 minutes vs voting right away?

You're right — I misspoke. The reverse auction sends those early rewards back to the pool, not the author. As for bots waiting exactly 5 minutes: there's no downside anymore, which is why many do exactly that. The 5-minute window mostly just delays bot voting slightly rather than stopping it.