I'm curious whether this can actually be considered to be a "CAPTCHA" in the true sense of the term. It doesn't seem to be intended to "tell computers and humans apart", but rather to force the client computer (not the human user) to do some work in order to slow down DOS attacks.
Of course slowing down DOS attacks is a great goal in itself, and it's very often what captchas have been (ab)used for, but it doesn't seem to me to replace all or most use cases for a captcha.
In particular, since it can be completed by an automated system at least as easily as by a human, it doesn't seem like it would limit spambot signups or spambot comment or contact form submissions in any meaningful way.
Of course slowing down DOS attacks is a great goal in itself, and it's very often what captchas have been (ab)used for, but it doesn't seem to me to replace all or most use cases for a captcha. In particular, since it can be completed by an automated system at least as easily as by a human, it doesn't seem like it would limit spambot signups or spambot comment or contact form submissions in any meaningful way.
Or am I misunderstanding, @realaravinth?