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Issue description

Hi.

I have spent the last few days trying to understand why my zsh startup time become so slow when I converted my from my usual prezto config to using home manager. I went from a startup time measured as such:

# $SHELL = /nix/store/apkv2fv89zd0knfi1l8pfyxl36xw3kzr-zsh-5.8/bin/zsh
for i in $(seq 1 10); do /usr/bin/time $SHELL -i -c exit; done

of around 0.10 seconds to 0.41 seconds using the same modules and config! 🐌

To make sure I was making a fair comparison, I tried to make a minimal reproducible example by comparing the exact same revision of prezto with the default options - one configured with home manager and the other per prezto's installation instructions.

With default modules I get 0.21 s startup with home manager.

I copied the prezto derivation from my local nix store to my ~/.zprezto (cp -R /nix/store/shwbywl4c5fj4ivsm628ari4ivayyfwz-zsh-prezto-unstable-2021-06-02/share/zsh-prezto .zprezto ) and ran prezto's setup in ~/.zprezto.

setopt EXTENDED_GLOB
for rcfile in "${ZDOTDIR:-$HOME}"/.zprezto/runcoms/^README.md(.N); do
  ln -s "$rcfile" "${ZDOTDIR:-$HOME}/.${rcfile:t}"
done

I still get 0.21 s as expected (it should be the same configuration, more or less), howerver I noticed that runcoms/zshrc is not sourcing ~/.zprezto/init.zsh but the init.zsh from the nix store, so I changed that:

# Source Prezto.
if [[ -s "${ZDOTDIR:-$HOME}/.zprezto/init.zsh" ]]; then
  source "${ZDOTDIR:-$HOME}/.zprezto/init.zsh"
fi

and re-ran the benchmark and then got excellent performance again at 0.10s, what is going on? 🤔

Of course I realise that the init.zsh in the nix store and the init.zsh in ~/.zprezto will actually load the modules from different locations, from the nix-store's prezto /modules and from ~/.zprezto/modules respectively. But the contents are identical!

Does anybody know what is going on here and has anyone experienced anything similar?

Furthermore, I tried to optimise the config a bit, and if I remove the completion module from my home-manager config, I get really good performance again at 0.09s which is identical to what I get with "vanilla" prezto (also without the completion module).
It also appears to be the case that the more other modules are loaded, the more the completion module contributes to a slower start up time. I do not see this behaviour, at least as clearly, with a vanilla prezto setup.

Maintainer CC

@anna328p
@pinpox

System information

- system: `"x86_64-darwin"`
 - host os: `Darwin 19.6.0, macOS 10.15.7`
 - multi-user?: `no`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.3.14`
 - channels(axel): `"home-manager, nixpkgs-21.11pre306170.4d3e13e51b6"`
 - nixpkgs: `/Users/axel/.nix-defexpr/channels/nixpkgs`