Came here to ask the same. Long time ohmyzsh user, and wondering what new features / benefits Starship has.
Most shells can probably do everything this can as well and if you're already familiar with the archaic syntax there is probably limited use for you.
For an idea, here is my current config https://github.com/LukasKnuth/dotfiles/blob/main/zsh/.config...
I also expect it has everything that you have in your prompt so is a direct replacement without losing anything.
1) I don't recall having a problem with prompt speed.
2) Are there any benchmarks?