Today, we are officially introducing a proof-of-work (PoW) defense for onion services designed to prioritize verified network traffic as a deterrent against denial of service (DoS) attacks with the release of Tor 0.4.8.
This is a new PoW protocol that will be used to defend against DDOSs
For anyone who comes by and just skims this, this is not cryptocurrency and it is not a captcha, it's just additional computation the browser will do in the background to prove it's a real human operated device.
They say it takes roughly 5ms on a fast computer and 30ms on a slow one, but on a DDOS attack would scale those up to a minute of work per connection.