Profiling .zshrc

If your terminal takes a long time to show the first prompt, it is because sourcing your .zshrc is slow.

.zshrc is a shell script that runs every time you open a new terminal. It is a hidden file in the home folder: ~/.zshrc It is used by package managers and other shell tools to configure themselves before hand. Consequently, you will see a lot of conda, nvm, etc. in there.

Run source is the equivalent of import. So source ~/.zshrc will re-load your shell.

Trivia: The "rc" stands for "RUNCOM" according to wikipedia Wikipedia. Other shells like bash also have .bashrc files.

How to Profile

Time stamped echos:

echo "Start $(date +%s%N)ns" # nanoseconds, 9 places after the second
# ... suspects go here ...
echo "end   $(date +%s%N)ns"

time [utility] is usefull for timing the execution time of a single function. time sh test.sh with a suspect line in test.sh will print the execution time.

To print a nice table with breakdown of time consuming functions use (scroll to the top of the output):

zmodload zsh/zprof
# zshrc here
zprof

Warning: zprof is wierd and actually adds up the number of calls if you run source ~/.zshrc again in the same terminal session.

Comment out the thing that is taking up time, and see how fast the startup is. Then decide whether to remove it or speed it up or keep it.

# source $ZSH/oh-my-zsh.sh # 90% of my time

What to look for

source means a whole additional script is being run, which could have plenty of time consuming stuff hidden inside. Btw. . is an alias for source so if you see a . "script.sh", make sure to go after it.

Package managers. Whereever there is a package manager, there will be source and other time consuming stuff.

  • Install lazy loading plugin from here. It uses lazy loading (loads only once needed.)
  • conda initialization is infamously slow. See this reddit comment for a lazy loader
  • nvm is a common suspect. Google how to lazy load it as well.

Terminal customizers, such as oh-my-zsh. They are the main consumers of loading time, and and supposed to be. Look into tweaking them or using a different one if they are to slow. Google possible solutions here is one for oh-my-zsh.

From the Profiler output

Look at the top 10 lines of the zprof output.

compdump and compinit and compaudit are shell completions (when you press tab and it show you available commands). They are the the heaviest suspect. They run only once, and remove them if possible. I have found them being run multiple times on multiple occasions.

  • Open the .zshrc file in a text editor and Ctrl+F for compinit and just the word completions. Even if compinit is not in the file, package managers often attach comments.
  • Not all completions are guilty. For example [ -s "/Users/me/.bun/_bun" ] && source "/Users/me/.bun/_bun" (bun's completion) doesn't call compinit and is very fast when measured with time or echo "$(date +%s%N)"

What Not to Look For

export is fast. Unlike source, export doesn't execute anything.

Powerlevel10k is famously fast.

More Resources

  • https://www.mikekasberg.com/blog/2025/05/29/optimizing-zsh-init-with-zprof.html
  • https://github.com/romkatv/zsh-bench?tab=readme-ov-file#summary
  • https://www.dribin.org/dave/blog/archives/2024/01/01/zsh-performance/
  • https://kasad.com/blog/zsh-profiling/
  • https://alextheobold.com/posts/profiling_zsh/