Well it's up to clients to generate the proof of work required by either relays or other clients fetching notes, and to implement filtering for follows based on a proof of work if the user desires.

It's a difficult problem, because you can't have user defined PoW thresholds that their follows don't know about, it's really best filtered by relays. Say I have a PoW requirement that the note id must start with 3 0s, but a follow produces them starting with 2, I won't get his notes. If I trust a relay though to require it and if there's some way for a relay to tell a client what it's PoW requirement is, then it's not a problem. PoW doesn't really affect your curated feed positively, it affects spam to relays, and if youre viewing a global feed, it can be used client side to hide spam at the cost of hiding basically everything from relays that don't require it.

As far as phones doing the PoW, that's not really that big of a deal. Some relays might require a high threshold to reduce spam, and if a user is incapable of producing it in a timely manner he can just publish to other relays, and if I'm big on following him I'll fetch his posts there. That relay might have a spammy global feed but as long as you're only fetching your follows from it it doesn't affect you. 

 I hear you, and I personally think if the relays filter at a low threshold it should be fine, but if they up that, then you'd likely want a majority of the client devs (including mobile) to be on-board in education of whats going on