Will my iPhone heat up even more with PoW?

they do but only because clients that filter PoW are fringe. If everyone does it, spammers will create a nostr note miner asic and then it is an arms race (somewhat joking). PoW definitely provides rate limiting. for example, if kind 0's were pow, then reply guy would have to shift strategies. I ...

It should be enough proof of work that it won't bother users. Maybe about 1 second delay. But that should be enough to start making spam expensive, right?

No they do not