How much resources are we talking about here? If it's 3% of your CPU usage for 2 seconds, you're really going to have an issue with that?
Whatever solution should be negligible for you, but costly for a botfarm.
Here's a live example, not exactly onerous: https://demo.mcaptcha.org/widget/?sitekey=pHy0AktWyOKuxZDzFfoaewncWecCHo23
(Obviously in Lemmy's case you wouldn't have the additional unecessary checkbox)
It seems like a no-brainer for me. Limits bots and provides a small(?) income stream for the server owner.
This was linked on your page, which is quite cool: https://crypto-loot.org/captcha