[Hnrobert42]

Maybe, the proof of work can run in the background.

[btown]

Or it can run as part of a checkout wizard's "verifying your browser and processing your payment, don't close your tab" step.

[mattstir]

At which point it exists solely to punish real human users? What scraper bot is going through checkout?

[BenjiWiebe]

The credit card tester bots go through the checkout process.

PoW wouldn't be a big issue for them though since their volume is much lower.