It's not perfect, but maxing a single core for 20 seconds on an older smartphone is a necessary evil for this kind of captcha.
The alternative: loading a third party script and multiple images (~2MB) to label for ReCAPTCHA and spending time performing the task also takes some battery (and mental) power.
The alternative: loading a third party script and multiple images (~2MB) to label for ReCAPTCHA and spending time performing the task also takes some battery (and mental) power.