There's also the Anubis idea where your PoW is persistent until your IP address or session cookie changes, so you get to skip PoW in exchange for making yourself identifiable, which means the PoW can then be ramped up to take a couple of minutes.

I don't use Anubis though. I just make my site not take five seconds to render a page so bots can overload it easily? It's not actually that hard?