I tried to implement PoW in browser for the same concept. I think it's probably at least marginally useful, but practically you're limited by WebCrypto I think,

I tried to implement PoW in browser for the same concept. I think it's probably at least marginally useful, but practically you're limited by WebCrypto I think, which outperformed anything I could find in pure JS or WASM. The disadvantage of WebCrypto is that there's a limited set of algorithms you can use and also, understandably, calculating hashes is async, so if you want a lot of rounds, you'll spend a lot of time jumping in and out of the event loop. It's still probably a useful speed bump or price increase for expensive operations: what might cost a few seconds on a typical phone or desktop is probably at least enough to act as a speed bump for attackers to perform it quickly (especially combined with other measures like throttling and progressively increasing PoW difficulty for an IP.) Maybe WebGPU could change this, but I'm weary of relying on all users having a fast GPU enabled in their browser just for this use case. Though thinking about it, I wonder if there is a hybrid: start with a difficulty that's just a few seconds for CPU/WebCrypto and ramp up quickly, but also support WebGPU where possible so that web users on abusive connections may still succeed? I am not sure though, I guess this depends on the feasibility of using WebGPU and etc.