The problem I see is that bots are usually faster to solve POW (multi-cpu, 24x7 operation) vs a small ARM mobile phone. Even if it was on par as in your example, the bot messages would still overwhelm normal user posts..

a bot will spam as fast as it can, for as long as it can. If it wanted to it could pre-calculate post dated messages in one huge batch and then send them out even faster.. things like that, just seems like it would hurt normal users, but not the bots.

I think the right idea too for nostr, is do the pow once per key. The most annoying of bots create a new key Everytime to bypass any restrictions or mute lists.. doing it per key would give normal users a leg up because they don't have a need for thousands of keys..

I still think lightning is a good measure of pow, because it's more instantaneous, most users would prefer not to wait around and are only on mobile.. (and when you think about it this is what Bitcoin is, it's pow, so in theory humans have it and bots will struggle to have it because spending it ruins their usual game of high volume per mark)

I could see supporting pow for those that don't want to do lightning, but they would need to spend a good 3 hours or so, maybe even a day, on a pow key during their onboarding.. which would still be easier to game for a bot, that could be sitting on many keys to deploy at once in a spam campaign.