reftex-save-parse-info

reftex-save-parse-info is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, save information gathered with parsing in a file.
The file MASTER.rel in the same directory as MASTER.tex is used to save the
information. When this variable is t,
- accessing the parsing information for the first time in an editing session
will read that file (if available) instead of parsing the document.
- exiting Emacs or killing a buffer in `reftex-mode' will cause a new version
of the file to be written.

You can customize this variable.