Speeding Up My Zsh Startup by 81%: From 640ms to 120ms

The Problem

My terminal was slow. Really slow. Every time I opened a new shell, there was a noticeable lag before my prompt appeared. Admittedly this was a papercut, but it made me feel like my laptop was slowing down. I don’t really want to upgrade right now, and I don’t really want to go scorched earth and wipe my machine to start again to get a fresh, fast terminal. So I decided to optimize my existing setup.

I jumped straight into fix-it mode. Let’s start with zoxide, that must slow down things, right? So I disabled it, which promptly broke tab completion with errors like __zoxide_z_complete: command not found. Great, now I have broken tab completion and a slow terminal. Fantastic.

I fell into the classic trap of “ready, fire, aim”. I made changes without measuring first.

So I needed to do what I should’ve from the start, I added profiling to figure out where all that time was actually going. 🤦

# Add to the top of ~/.zshrc
zmodload zsh/zprof

# Add to the bottom
zprof

As usual, the results were not what I expected.

The Diagnosis

Startup time: 640ms 🐌

Here’s what zprof revealed:

That’s nice, but what does it actually mean? I had no idea what compinit or compdef were doing, and I had no idea why they were taking so long. I just knew that zoxide was the last thing I added, so it must be the problem, right?

A Different Kind of Pair Programming

I usually use GitHub Copilot for my day-to-day coding, but this week I decided to try out Claude Code for this optimization problem. The difference was notable.

I pasted my zprof output into Claude and asked it to analyze the results. Within seconds, it identified the major culprits, and zoxide wasn’t one of them.

1. compinit was being called twice—once by Antigen, once by Oh-My-Zsh
2. The plugin manager itself was eating 300ms+
3. I had previously put in a quick fix for some plugins not working, and the full Oh-My-Zsh was loading when I only needed a handful of features
4. I was loading a built in and external fzf plugin

Two minutes with zprof and a dash of AI would have saved me 20 minutes of wrong guesses. Measure first, tinker second.

It suggested migrating from Antigen to Zinit. Ironically, I’d tried Zinit a few years ago when I first set up my terminal with Antigen but couldn’t get it configured quite right so I parked it. Now with the power of AI, this would be different.

Here’s what made it valuable: this wasn’t just handing off the problem. I asked questions throughout:

• “Why is compinit being called twice?”
• “What’s the difference between these fzf plugins?”
• “Can we keep the OMZ git plugin features I use?”

We debugged the ZLE warnings together, and I understood each change we made. It felt more like pair programming than autocomplete — and I ended up with a configuration I still understand.

What Actually Worked

Step 1: Replace Antigen with Zinit

Antigen is convenient but slow. Zinit is a modern, faster alternative. This was a great suggestion from Claude, and it did a great job of converting my setup in a couple of minutes with only a few back-and-forths to fix edge cases.

# Zinit installation (faster than antigen)
ZINIT_HOME="${XDG_DATA_HOME:-${HOME}/.local/share}/zinit/zinit.git"
[ ! -d $ZINIT_HOME ] && mkdir -p "$(dirname $ZINIT_HOME)"
[ ! -d $ZINIT_HOME/.git ] && git clone https://github.com/zdharma-continuum/zinit.git "$ZINIT_HOME"
source "${ZINIT_HOME}/zinit.zsh"

Savings: ~200ms

Step 2: Load Only What You Need from Oh-My-Zsh

Instead of loading the entire Oh-My-Zsh framework, I cherry-picked just the libraries and plugins I actually use. I had a list of plugins at the top of my ~/.zshrc that I wanted to keep, so I made sure to include those. I wasn’t aware that I had added a call in the prompt that caused the entire framework to load.

# Load OMZ libs we need (not the whole thing)
zinit snippet OMZL::git.zsh
zinit snippet OMZL::theme-and-appearance.zsh
zinit snippet OMZL::async_prompt.zsh  # Required by git plugin

# Load OMZ plugins
zinit snippet OMZP::git
zinit snippet OMZP::brew
zinit snippet OMZP::direnv
zinit snippet OMZP::eza
# Removed OMZP::fzf - using fzf-tab instead

This eliminated the overhead of loading unused OMZ plugins and prevented the double compinit call.

Savings: ~150ms

Step 3: Loading 3rd Party Plugins

# Fast syntax highlighting and autosuggestions
zinit light zsh-users/zsh-syntax-highlighting
zinit light zsh-users/zsh-autosuggestions

# fzf-tab for better tab completion
zinit light Aloxaf/fzf-tab
# Don't load fzf-tab-source - it has bugs and we don't need it

Key learning: The OMZP::fzf plugin was causing harmless but annoying ZLE warnings. Since we’re using the superior fzf-tab plugin, we don’t need it.

Savings: ~50ms + eliminated warnings

Step 4: Load Completions Once

Instead of letting multiple plugins call compinit, do it once explicitly:

# Load completions efficiently
autoload -Uz compinit
compinit

Savings: ~180ms (from eliminating duplicate call)

Step 5: Initialize Tools Properly

Re-enable zoxide (it wasn’t the problem after all!):

# Initialize tools
eval "$(starship init zsh)"
eval "$(fnm env --use-on-cd --shell zsh --version-file-strategy=recursive)"
eval "$(zoxide init zsh)"

The Results

New startup time: 120ms 🚀

New zprof breakdown:

num  calls                time                       self            name
-----------------------------------------------------------------------------------
 1)  508          16.88     0.03   19.72%     14.13     0.03   16.50%  :zinit-tmp-subst-alias
 2)    2          11.05     5.53   12.91%     11.05     5.53   12.91%  compaudit
 3)    7          12.58     1.80   14.70%      9.23     1.32   10.78%  .zinit-load-snippet
 4)    1          19.04    19.04   22.24%      7.99     7.99    9.33%  compinit
 5)    4          43.43    10.86   50.73%      6.23     1.56    7.28%  .zinit-load-plugin
 6)    1           5.07     5.07    5.92%      5.07     5.07    5.92%  _fnm_autoload_hook

Cleanup: Removing Old Caches

After migrating to Zinit, I cleaned up the old Antigen cache:

#!/bin/bash
# Remove old antigen cache
rm -rf ~/.antigen

# Clean old completion dumps
rm -f ~/.zcompdump*

# Regenerate fresh cache
zsh -i -c "autoload -Uz compinit && compinit -C"

The Complete Configuration

Here’s the final ~/.zshrc:

# used for debugging and profiling - uncomment if needed
# zmodload zsh/zprof

# Zinit installation (faster than antigen)
ZINIT_HOME="${XDG_DATA_HOME:-${HOME}/.local/share}/zinit/zinit.git"
[ ! -d $ZINIT_HOME ] && mkdir -p "$(dirname $ZINIT_HOME)"
[ ! -d $ZINIT_HOME/.git ] && git clone https://github.com/zdharma-continuum/zinit.git "$ZINIT_HOME"
source "${ZINIT_HOME}/zinit.zsh"

# Load OMZ libs we need (not the whole thing)
zinit snippet OMZL::git.zsh
zinit snippet OMZL::theme-and-appearance.zsh
zinit snippet OMZL::async_prompt.zsh

# eza config - must be set BEFORE loading the plugin
zstyle ':omz:plugins:eza' 'dirs-first' yes
zstyle ':omz:plugins:eza' 'git-status' yes
zstyle ':omz:plugins:eza' 'header' yes
zstyle ':omz:plugins:eza' 'icons' yes

# Load OMZ plugins
zinit snippet OMZP::git
zinit snippet OMZP::brew
zinit snippet OMZP::direnv
zinit snippet OMZP::eza

# Fast syntax highlighting and autosuggestions
zinit light zsh-users/zsh-syntax-highlighting
zinit light zsh-users/zsh-autosuggestions

# fzf-tab
zinit light Aloxaf/fzf-tab

# Load completions efficiently (after prompt is ready)
autoload -Uz compinit
compinit

# Initialize tools - these need to load synchronously
eval "$(starship init zsh)"
eval "$(fnm env --use-on-cd --shell zsh --version-file-strategy=recursive)"
eval "$(zoxide init zsh)"

export HOMEBREW_AUTO_UPDATE_SECS="86400"

# history setup
HISTFILE=$HOME/.zhistory
SAVEHIST=1000
HISTSIZE=999
setopt share_history
setopt hist_expire_dups_first
setopt hist_ignore_dups
setopt hist_verify

# Aliases
alias code=code-insiders
alias d=dotnet
alias db="dotnet build"
alias dw="dotnet watch"
alias dr="dotnet run"
alias dt="dotnet test"
alias gprune='git branch --merged origin/$(git_main_branch) | xargs git branch -d'
alias zshconfig="code ~/.zshrc"

# pnpm
export PNPM_HOME="/Users/matt/Library/pnpm"
case ":$PATH:" in
  *":$PNPM_HOME:"*) ;;
  *) export PATH="$PNPM_HOME:$PATH" ;;
esac

# RVM
export PATH="$PATH:$HOME/.rvm/bin"

# Aspire
export PATH="$HOME/.aspire/bin:$PATH"

# vscode integration
[[ "$TERM_PROGRAM" == "vscode" ]] && . "$(code --locate-shell-integration-path zsh)"

# used for profiling only 
# zprof

Try It Yourself

1. First, profile your current setup:

   # Add to top of ~/.zshrc
   zmodload zsh/zprof
   
   ...
   
   # Add to bottom of ~/.zshrc
   zprof

2. Measure your startup time:

   for i in {1..5}; do /usr/bin/time zsh -i -c exit 2>&1 | grep real; done

3. Review the zprof output to identify your actual bottlenecks (don’t guess like I did!)

4. Optimize based on what the data shows

5. Measure again to verify your improvements

Wrapping Up

Your terminal is where you spend a huge chunk of your dev time. If yours feels sluggish, don’t guess — profile it first. My intuition was completely wrong, and two minutes with zprof saved me from going down the wrong rabbit hole.

Data > Intuition. Always.

Happy profiling! ⚡