Benchmarking the time zsh takes to start up (though I believe it is more accurate with my own benchmark script for the simple absolute time output).
What is really useful however is the percentage, i.e. relative time output of individual function calls by zsh itself.
What it boils down to is putting zmodload zsh/zprof on top of your startup (e.g. .zshrc) and zprof at the very end.
When creating a new shell this will nicely output whatever happens in between.