Avoid processing the same buffer multiple times.

zsh-syntax-highlighting.zsh

@@ -137,6 +137,10 @@ _zsh_highlight-string() {
 
 # Recolorize the current ZLE buffer.
 _zsh_highlight-zle-buffer() {
+  # Avoid doing the same work over and over
+  [[ ${ZSH_PRIOR_HIGHLIGHTED_BUFFER:-} == $BUFFER ]] && return
+  ZSH_PRIOR_HIGHLIGHTED_BUFFER=$BUFFER
+
   setopt localoptions extendedglob bareglobqual
   local colorize=true
   local start_pos=0