Performance Tuning

zflash is fast by default. These steps take a typical setup from tens of milliseconds to single digits. They're listed roughly by impact.

Measure as you go:

zflash bench 20

Defer almost everything

Anything in stage 1 delays the prompt. Move to defer everything that isn't needed to draw the prompt: the oh-my-zsh base library, fzf-tab, tool integrations, plugins, aliases and functions. With something deferred, compinit moves after the prompt too.

Capture init output at build time

Replace eval "$(tool init zsh)" with zflash eval. For starship, also capture PROMPT2, which otherwise forks starship prompt --continuation on every start (about 4 ms):

zflash eval 'starship init zsh --print-full-init | grep -v "^PROMPT2="'
zflash eval 'print -r -- "PROMPT2=${(q+)$(starship prompt --continuation)}"'

Move your rc files into the spec

Files listed in the spec are byte-compiled and can be staged. source lines in .zshrc are parsed every time and always block the prompt. See Your own files.

macOS: skip the global rc files

On macOS, /etc/zprofile runs path_helper and /etc/zshrc runs locale: two forks, about 3 ms per shell. Skip both with one line:

~/.zshenv

setopt no_global_rcs

Then provide what those files set up yourself:

• PATH: the entries from /etc/paths and /etc/paths.d/*, written as a static list. Keep $path so inherited entries survive:

  ~/.zsh/exports

  typeset -U path
  path=(/opt/homebrew/bin /opt/homebrew/sbin /usr/local/bin /usr/bin /bin /usr/sbin /sbin $path)

• setopt COMBINING_CHARS, for correct display of accented characters.

• bindkey -e, if your EDITOR contains "vi" and you want emacs key bindings.

Keep login and non-login shells consistent

The compinit cache is keyed on the exact fpath. If ~/.zprofile changes fpath (or PATH entries that affect it), login and non-login shells see different fpaths and rebuild the dump each time you switch. Keep fpath changes in the spec, not in ~/.zprofile.

Compinit timing

ZFLASH_COMPINIT	Trade-off
defer (default)	Fastest prompt. Tab in the first few ms uses basic completion.
sync	Full completion immediately; adds the compinit cost to startup.

Verify deferred loading works

zsh-defer hides errors from the deferred stage. If a plugin seems not to load, see Troubleshooting.