The default $PROMPT_COMMAND is

$ echo $PROMPT_COMMAND
_z --add "$(command pwd -P 2>/dev/null)" 2>/dev/null

For me (on Cygwin, Windows 8.1), the default configuration makes it so that if I hold down the Enter key in Bash, the prompts start to lag behind. Overall, the prompt feels slow.

So I changed it to

$ echo $PROMPT_COMMAND
(_z --add "$(command pwd -P 2>/dev/null)" 2>/dev/null &)

This seems to mostly fix the issue. The z datafile is updated in the background, silently.

Is there any harm in doing this or reason why this isn't the default?