My personal worst experience with captchas is Vultr: every time I go to their page from my Linux desktop, I need to solve a recaptcha. And if I don't do it within ~3 seconds, the page reloads and I need to start over again. I bet there are more robots than humans who can solve it in such a short time. You need to read the instructions, pattern match 9 photos... twice... in 3 seconds. It doesn't happen when I visit the Vultr's page from my iPhone though. When I get a spare moment, I will move my VPS to another, less obnoxious provider (maybe even Amazon).