I just discovered this and haven't tried it yet. The reason why this is so much faster seems to mostly be that it uses https://github.com/romkatv/gitstatus (by the same author) as a daemon to get git status info.
The classical way of running a dozen git commands gets really slow in complicated circumstances. I think previously you had to either limit displayed git info or update the prompt contents asynchronously to get a reasonable latency in a Zsh prompt.
Indeed, gitstatus is one of the reasons Powerlevel10k is faster than all other ZSH themes. It also uses a patched version of libgit2 that is 4 times faster than the original. There is some info here: https://www.reddit.com/r/linux/comments/b0suyu/powerlevel10k....
Powerlevel10k also uses advanced ZSH. Explained in https://www.reddit.com/r/zsh/comments/b1xxuj/abusing_zsh_par....
There is also information on https://github.com/romkatv/powerlevel10k/ that probably didn't exist when you've discovered Powerlevel10k.