Araaha
Extreme ZSH Performance.
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This blog post is for those looking to improve their ZSH startup time. With a clean .zshrc, ZSH is quite fast. But once you start adding various things to it, it can slow down dramatically.

TLDR: Organize your .zshrc effectively and use zsh-defer.

Performance Analysis

While you could use everything from zprof to hyperfine and time, the most comprehensive way is with zsh-bench. Just run

git clone https://github.com/romkatv/zsh-bench ~/zsh-bench && cd zsh-bench

in your terminal. Running the zsh-bench script outputs this:

==> benchmarking login shell of user araaha ...
creates_tty=1
has_compsys=1
has_syntax_highlighting=1
has_autosuggestions=1
has_git_prompt=0
first_prompt_lag_ms=237.871
first_command_lag_ms=263.405
command_lag_ms=40.244
input_lag_ms=11.732
exit_time_ms=90.718

on my laptop (AMD Ryzen 5 5500U).

The lines we want to focus on are the latter four:

first_prompt_lag_ms=237.871
first_command_lag_ms=263.405
command_lag_ms=40.244
input_lag_ms=11.732

The line first_prompt_lag_ms measures the time until the prompt appears. Similarly, first_command_lag_ms measures the time until you can start typing a command. command_lag_ms measures the time between the execution of a command and the first moment a prompt appears thereafter. Lastly, input_lag_ms measures the time between keystrokes. As reference, using a totally clean .zshrc, the following is printed out:

==> benchmarking login shell of user araaha ...
creates_tty=0
has_compsys=0
has_syntax_highlighting=0
has_autosuggestions=0
has_git_prompt=0
first_prompt_lag_ms=12.648
first_command_lag_ms=13.152
command_lag_ms=0.537
input_lag_ms=0.357
exit_time_ms=10.877

Organizing ZSH

Originally, I had a .zshrc with hundreds of lines. I managed to reduce it to just 32. You can use $ZDOTDIR to place your zsh config in a dedicated folder. In any case, I have mine exported as

export ZDOTDIR=/home/araaha/.config/zsh

in /etc/zsh/zshenv. That way, $HOME is kept uncluttered. The layout is as follows:

zsh
 ├─ modules
 │  ├─ bindkeys.zsh
 │  ├─ zstyle.zsh
 │  ├─ options.zsh
 │  ├─ prompt.zsh
 │  ├─ compinit.zsh
 │  ├─ zvm.zsh
 │  ├─ zoxide.zsh
 │  ├─ exports.zsh
 │  ├─ tty.zsh
 │  ├─ history.zsh
 │  └─ aliases.zsh
 └─ plugins
    ├─ zsh-completion-generator
    ├─ zsh-defer
    ├─ functions.zsh
    ├─ fast-syntax-highlighting
    ├─ zsh-autosuggestions
    ├─ fzf.zsh
    └─ zsh-vi-mode

Every plugin I have is placed in plugins and everything else in modules. For example, exports.zsh includes exports I have. Similarly, aliases.zsh includes aliases I have.

Zsh-defer

Now, rewriting your .zshrc is straightforward. Personally, the first three lines in my .zshrc are:

export ZSH="$HOME/.config/zsh"
export PLUG="$ZSH/plugins"
export MOD="$ZSH/modules"

We can start sourcing our modules and plugins and use zsh-defer as much as possible. zsh-defer allows us to defer ZSH commands. It has to come before sourcing anything you want to defer. In some cases, you'll end up breaking things by deferring. In my case, I can't defer the zsh-vi-mode plugin. I also don't defer my prompt, zsh opts and my history opts. In any case, the latter three don't siginificantly affect startup time.

By the end, I ended up with

export ZSH="$HOME/.config/zsh"
export PLUG="$ZSH/plugins"
export MOD="$ZSH/modules"

source "$PLUG/zsh-defer/zsh-defer.plugin.zsh"

#zvm must be above zsh-vi-mode
source "$MOD/zvm.zsh"
source "$PLUG/zsh-vi-mode/zsh-vi-mode.zsh"

zvm_after_init_commands+=("[[ -t 0 && $- = *i* ]] && stty -ixon")

source "$MOD/prompt.zsh"
source "$MOD/options.zsh"
source "$MOD/history.zsh"

zsh-defer source "$MOD/exports.zsh"
zsh-defer source "$MOD/aliases.zsh"
zsh-defer source "$MOD/bindkeys.zsh"

zsh-defer source "$MOD/zoxide.zsh"
zsh-defer source "$MOD/compinit.zsh"
zsh-defer source "$MOD/zstyle.zsh"

zsh-defer source "$PLUG/fzf.zsh"
zsh-defer source "$PLUG/functions.zsh"
zsh-defer source "$PLUG/zsh-autosuggestions/zsh-autosuggestions.zsh"
zsh-defer source "$PLUG/fast-syntax-highlighting/fast-syntax-highlighting.zsh"

which led to this result from zsh-bench:

==> benchmarking login shell of user araaha ...
creates_tty=0
has_compsys=0
has_syntax_highlighting=0
has_autosuggestions=0
has_git_prompt=0
first_prompt_lag_ms=84.171
first_command_lag_ms=92.650
command_lag_ms=39.395
input_lag_ms=11.381
exit_time_ms=13.126

The plugins that affect performance the most are zsh-autosuggestions and zsh-vi-mode. Without them, command_lag_ms is almost zero and first_prompt_lag_ms, first_command_lag_ms are respectively ~10-20ms.

Additional Optimizations

Compinit

compinit siginificantly affects startup time. We can cache it once a day to speed things up:

autoload -Uz compinit
if [ $(date +'%j') != $(date -r $ZSH/.zcompdump +'%j') ]; then
    compinit
else
    compinit -C
fi

Zcompile

Using zcompile on your plugins can have an impact. E.g., using zcompile on zsh-vi-mode reduces my startup time by ~10ms.

Git Prompt

Using an async git prompt will likely improve performance.

Oh-My-ZSH

OMZ has dozens of plugins, most of which you likely won't use. It's better to create your own config and only include plugins you regularly use.