Speed up zsh compinit by only checking cache once a day.

compinit.zsh

# On slow systems, checking the cached .zcompdump file to see if it must be 
# regenerated adds a noticable delay to zsh startup.  This little hack restricts 
# it to once a day.  It should be pasted into your own completion file.
#
# The globbing is a little complicated here:
# - '#q' is an explicit glob qualifier that makes globbing work within zsh's [[ ]] construct.
# - 'N' makes the glob pattern evaluate to nothing when it doesn't match (rather than throw a globbing error)
# - '.' matches "regular files"
# - 'mh+24' matches files (or directories or whatever) that are older than 24 hours.
autoload -Uz compinit 
if [[ -n ${ZDOTDIR}/.zcompdump(#qN.mh+24) ]]; then
	compinit;
else
	compinit -C;
fi;

@vincentbernat I solved the multiple shell problem by using a lockfile and a trap to remove it. This ensures that only one process is responsible for updating the file. Even though there's still a race condition, it's now almost impossibly small ¹

() {
	setopt local_options
	setopt extendedglob

	local zcd=${1}
	local zcomp_hours=${2:-24} # how often to regenerate the file
	local lock_timeout=${2:-1} # change this if compinit normally takes longer to run
	local lockfile=${zcd}.lock

	if [ -f ${lockfile} ]; then 
		if [[ -f ${lockfile}(#qN.mm+${lock_timeout}) ]]; then
			(
				echo "${lockfile} has been held by $(< ${lockfile}) for longer than ${lock_timeout} minute(s)."
				echo "This may indicate a problem with compinit"
			) >&2 
		fi
		# Exit if there's a lockfile; another process is handling things
		return
	else
		# Create the lockfile with this shell's PID for debugging
		echo $$ > ${lockfile}
		# Ensure the lockfile is removed
		trap "rm -f ${lockfile}" EXIT
	fi

	autoload -Uz compinit

	if [[ -n ${zcd}(#qN.mh+${zcomp_hours}) ]]; then
		# The file is old and needs to be regenerated
		compinit
	else
		# The file is either new or does not exist. Either way, -C will handle it correctly
		compinit -C
	fi
} ${ZDOTDIR:-$HOME}/.zcompdump 

---

For those who are new to some of this stuff, check out

• https://zsh.sourceforge.io/Doc/Release/Completion-System.html
  • For how compinit works
• https://zsh.sourceforge.io/Doc/Release/Expansion.html#Filename-Generation
  • For the (#q.N.....) syntax, search for "file access qualifier"

Footnotes

1. A second process would have to check for the file in between 2 (effectively) subsequent commands. if [ -f ${lockfile} ] and echo $$ > ${lockfile} ↩

@thefotios my solution is already handling this case (but my message wasn't quite clear on that, so I understand you thought this was not the case). This is the purpose of the ln command (which is atomic, so no race condition).

I commented out these lines in my .zshrc which sped it up a lot:

if type brew &>/dev/null; then
    FPATH=$(brew --prefix)/share/zsh-completions:$FPATH

    autoload -Uz compinit
    compinit
if

As this comment above pointed out, oh-my-zsh already runs compinit.