Shell startup: from 5s to 140ms

June 11, 20262 min read

zsh
performance

My zsh shell took over 5 seconds to start. I timed it, found the bottlenecks, and got it down to ~140ms. Here’s what worked.

Measuring

Before changing anything, measure:

time zsh -i -c exit

Run it 3-5 times and take the median. On my machine this was consistently over 5 seconds.

The bottlenecks

NVM (~300ms)

NVM sources a full shell script on every shell startup, even if you never use node in that session. The fix is lazy-loading: wrap the commands in functions that load the real NVM on first use:

nvm() { unset -f nvm node npm npx; [[ -s "$HOME/.nvm/nvm.sh" ]] && . "$HOME/.nvm/nvm.sh"; nvm "$@"; }
node() { unset -f nvm node npm npx; [[ -s "$HOME/.nvm/nvm.sh" ]] && . "$HOME/.nvm/nvm.sh"; node "$@"; }
npm() { unset -f nvm node npm npx; [[ -s "$HOME/.nvm/nvm.sh" ]] && . "$HOME/.nvm/nvm.sh"; npm "$@"; }
npx() { unset -f nvm node npm npx; [[ -s "$HOME/.nvm/nvm.sh" ]] && . "$HOME/.nvm/nvm.sh"; npx "$@"; }

The first call to any of these unsets all four functions, loads NVM for real, then forwards the call. Every subsequent call goes through the real NVM at native speed.

virtualenvwrapper (~200ms)

Same problem, same fix:

_load_venvwrapper() { unset -f workon mkvirtualenv rmvirtualenv; source /opt/homebrew/bin/virtualenvwrapper.sh; }
workon() { _load_venvwrapper; workon "$@"; }
mkvirtualenv() { _load_venvwrapper; mkvirtualenv "$@"; }
rmvirtualenv() { _load_venvwrapper; rmvirtualenv "$@"; }

compinit deduplication

Oh My Zsh calls compinit once, but some plugins and custom configurations trigger it again. Each compinit call scans the entire fpath for completion functions. Making sure it only runs once saved another noticeable chunk.

Tools that didn’t slow things down

These are instant and worth keeping:

zoxide (eval "$(zoxide init zsh)"): negligible
Powerlevel10k instant prompt: it improves perceived startup by rendering the prompt before the rest of .zshrc finishes
fzf: the plugin is lightweight

Result

Before:  5.2s average
After:   ~140ms average

The shell now opens faster than my fingers can move to start typing. The key insight: most startup cost comes from tools eagerly initializing “just in case.” Lazy-loading defers that cost to the moment you need it. For many sessions, that moment never comes.

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