I am using zsh 4.3.17.

This series of initialization (with the attached, admittedly rather
big DIRCOLORS.256), makes auto_cd very slow.

zsh -f
setopt AUTO_CD
eval $(dircolors -b ~/DIRCOLORS.256)
autoload -U compinit; compinit
zstyle ':completion:*' list-colors ${${(s.:.)LS_COLORS}/\*/\=\*}
zstyle ':completion:*' group-name ''

What I am trying to achieve with the substitution in the list-colors
style is to make the explicit coloring of files with file endings take
precedent over, say, executables.

With the above lines loaded, the following completion takes over a
full second on my Linux system:

/us<tab>

whereas completion after an explicit cd is nearly instantaneous:

cd /us<tab>

It seems rather odd to me that coloring would render this so slow. I
suppose the delay is caused by zsh trying to color some list of
possible completions, but why would it be so very slow?