Very good point!

Accessibility is a critical to mCaptcha. In fact, Codeberg is trying out mCaptcha purely because of its more accessible[0]. That said, it is possible to choose a difficulty factor very high to deny access to folks with older, slower devices. A survey to benchmark mCaptcha performance on devices in the wild is WIP[1]. I hope it will provide insights to help webmasters integrating mCaptcha to select difficulty factors that work for their visitors.

[0]: https://codeberg.org/Codeberg/Community/issues/479#issuecomm...

[1]: https://github.com/mCaptcha/survey

The problem is that mCaptcha allows you to deny access to older, slower devices, but does nothing to deny access to bots. Which can likely run optimized implementations of the PoW which run hundreds of times faster than the web based version.