Here's an example of the "select all things until none are left" captcha slowdown (that I've encountered personally as well):
I've seen that happen before (rarely) on Tor, but a small delay isn't very bothersome, and I've always been able to fix the
really bad delays by refreshing the page or getting a new Tor identity.
Elsewhere, someone suggested Solvemedia, but I think that all captchas based on reading text are actually
easier for high-quality OCR to solve than for humans. The photo-based ones give computers a major disadvantage over humans. I don't care very much if people use services like 9kw.eu to solve the captchas, since that at least has a cost, but if a captcha can be OCRed, then they have no cost at all, making them completely useless.
If someone has other suggestions for good captcha services/libraries, let me know. I don't really like using Google products, since Google's whole business model is spying on people. Though as I mentioned, I don't have much faith in captchas based on reading mangled text.
I am not sure how difficult to implement this via SMF would be, however would it be possible to have users attempt to login on /login.php then on a /login2.php page would check if the account attempting to be logged into meet a certain criteria
I thought about that, but it'd be a bit difficult to implement, and I'm not sure how much value it'd really provide. Even if a captcha isn't required in 99% of logins, the 1% of logins where it is required is going to screw up bots, etc.