I'm wondering if the Xcode simulator (without Xcode running) performs as well, my 2020 Intel MacBook Air has been incapable of running Safari in iOS smoothly for nearly all its life.
There’s no statement or action (such as banning menu-bar-only apps from the Store or even changing the APIs) supporting that Apple still wants menu bar items to be ephemeral.
No because that additional CSS file can only be fetched after the initial HTML file has been fetched, not simultaneously; thus requiring another roundtrip to the server. (HTTP/2 Server Push promised otherwise but it was never implemented correctly.)
Latency being much more constraining than bandwidth, embedding a stylesheet that’s small is nearly always the correct tradeoff.