Comment by gruez

>The assumption is that requiring 1 second of compute before allowing a page load will have more impact on the people requesting 1000 pages/sec than it will on consumers requesting 1 page every minute.

Which doesn't hold. A 1s delay, despite being inperceptible is still a cost to human users. After all, 3600 x 1s delay adds up to an hour. Worse yet, the typical human visitor will always have to solve a challenge, because chances are it's the first time they're visiting your blog or whatever. Someone scraping reuse a valid session for at least a few more pages.

There's also the problem of time. It might be tempting to think the cost of a challenge is 1M cpu cycles or whatever, but the cost is better modeled as two parts, a time cost (for the human) and a cpu cost (for the computer). A scraper might have to pay the cpu cost, but not the time cost, because they can have their scraper solve challenges in their sleep. Unless the user is utterly destitute, their time is worth far more than whatever the compute cost is.