Harry Brignull on CAPTCHAs and conversion rates:
Users were directed to the sign-up form direct from the homepage before they could interact with the product. As you can see, there was a CAPTCHA at the bottom of the form (powered by reCAPTCHA). With this design, they had a conversion rate of roughly 48%. They then removed the CAPTCHA, and it boosted the conversion rate up to 64%. In conversion rate lingo, that’s an uplift of 33.3%! They replaced the CAPTCHA with honeypot fields and timestamp analysis, which has apparently proven to be very effective at preventing spam while being completely invisible to the end user.
In order for CAPTCHAs to be useful and/or effective, they have to render text in a way that requires a human to stop and examine it closely (in order to prevent machines from reading it).
Slowing down a registration or commenting process makes it feel like more work for the user or potential user. It’s an increase in cognitive load.