bsky.app works absolutely fine in a web browser and I use it instead of the mobile application. (It's React Native, so they push a build to the web.)
I wouldn't call it "absolutely fine" by any means. It's way too slow to render for me to use (took about 30 seconds to render the submitted link on my phone, mastodon took about 10 seconds to render a toot). I'm beyond sick of slow web pages.
As for performance on my iPhone 15, on a 5G (not 5GUW) network, it loads in a second and a half; I just checked. I have some complaints with what RN does in terms of affordances but performance isn't a problem.