After fixing tmux, I looked at the shell running inside it. My zsh config had grown over a few years to about 30 plugins and Oh My Zsh snippets on top of zinit and Powerlevel10k. It felt a little heavy, but nothing stood out.

Measured properly, it was slow in three separate ways, each with its own cause:

This post covers how I measured each one, the fixes, and how I checked that the fixes didn’t change anything else. That check caught two mistakes of my own.