Caching is a bit dangerous, and most shells don't support asynchronous prompt updates. And cross-shell support seems to be a prime feature. Spending probably a lot of effort just for improving zsh experience might just not have been important enough yet.

The power of powerlevel10k is that it can draw the plain prompt immediately, and respond to keyboard immediately, and then backfill any advanced prompt information (and properly shifting the already added text to the right and wrapping it correctly). I know that zsh has support for this, and bash does not (powershell neither); my prime reason why I use zsh. Fish, I don't know how it handles prompts.

This is very powerful in large git repos etc. You're never waiting for the prompt. You just type immediately, w/immediate response. So, you only wait for the status information to show up when you need it. So you also don't pay an excessive penalty if you prefer overly chatty prompts.

AFAICT the lazy-loading and async-support tickets above talk about internal implementation details, timeout support (so cancelling stuff like a too slow status report and showing a simplified prompt). I may misunderstand that, I only gave it a quick glance.

Maybe starship already supports this for zsh, or not, but at least they don't promote it front and center.