24 is ok on a pixel 8 pro, less than 15 seconds. 20 is basically instant.

21

Is there any relays that require proof of work?

16-20

PoW cost is irrelevant. The Fed’s Helicopter Money has a negative marginal cost yet it’s printed the most money in history. You’re asking the price of blood while the body is already blue.

I joined the 21 pow club but after a couple weeks I gave up. I didn't see the point of it anymore. It uses up my phone resources. If the clients gave us the option to hide non pow notes then maybe I'd do it.