This challenge only comes up when you're using known Tor/Proxy IPs which is usually where DDOS attacks originate from. In terms of L7 attacks, the challenge is what limits its effectiveness before it hits your site. When facing large DDOS attacks or sophisticated ones, you don't have many good options beyond CloudFlare. Privacy is compromised eitherways when traffic has to go through someone else before you.

If you're facing extremely long load times for captchas, it is often better to actually just change the routing circuit.

I don't know if a very small number of people use Tor on this forum (it seems so to me), because for two days the forum was inaccessible to me through the same thing, and you are the first to complain about it. It is possible that for some reason you still have problems, I don't have them at all anymore, and someone didn't even have them yesterday.