They are computationally slow, no tricks. The purpose is to prevent attackers from calculating the plain passwords if they actually got access to the content of your DB. They are not that slow (let's say less than a second to calculate) - fast enough for your user to not notice , slow enough so that attacker can't do gazillion tries per second with their GPU or something.

As per DOS: rate limit per ip and account, and it's not an issue.