reftex-support-index

reftex-support-index is a variable defined in `reftex-vars.el'.
Its value is

t

Documentation:
Non-nil means, index entries are parsed as well.
Index support is resource intensive and the internal structure holding the
parsed information can become quite big. Therefore it can be turned off.
When this is nil and you execute a command which requires index support,
you will be asked for confirmation to turn it on and rescan the document.

You can customize this variable.