It is nonstandard for mobile apps to require a user to fill out a captcha to log in. Yet, almost all of our users on the mobile apps are required to do so. The captcha presents a big problem for our users; only 40% of those who open the create account form actually manage to create an account, and many get stuck filling in captcha after captcha incorrectly. It's not uncommon for us to get reviews where users ask how many captchas they have to fill in before they can register.
Let's find some other solution that satisfies our security needs (e.g. client-side throttling) whilst avoiding having to serve users captchas on mobile apps.