Yes, an iPhone CPU is usually fast enough not to drop frames but if you care about battery life it’s not competitive. Again, I’m not saying that VP9 is bad but rather that anyone serious needs to support both formats until AV1 has MPEG-4 levels of pervasiveness, especially if you’re publishing on the web.
No, if you're publishing on the web using VP9 is the better choice. VP9 is supported in more browsers and there's no point having the licensing hassle of H.265. And as Netflix found, VP9 outperforms H.265 by 12% with the right encoder:
