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> I think this is just what Google does when it thinks you're a "bot": i.e. they don't know who you are.

Maybe this is to prevent adversarial learning? If the images reload immediately, then the bot can learn (via a neural network) whether its solution was good or not. If there's a delay, it's the same, but the learning is slowed down by the same factor.

No idea if this is true, it just popped out of my head.

As I said below, this is what I thought too, but another HN'er pointed out that a bot could load up a thousand captcha's in parallel and just switch between them while waiting for the fade in.

Sure, they can, but putting a small delay forces an arms race on how big a cluster you need to operate those parallel bots and how cheaply you can offer a cracking service.

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