reftex-trust-label-prefix

reftex-trust-label-prefix is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, trust the label prefix when determining label type.
It is customary to use special label prefixes to distinguish different label
types. The label prefixes have no syntactic meaning in LaTeX (unless
special packages like fancyref are being used). RefTeX can and by
default does parse around each label to detect the correct label type,
but this process can be slow when a document contains thousands of
labels. If you use label prefixes consistently, you may speed up
document parsing by setting this variable to a non-nil value. RefTeX
will then compare the label prefix with the prefixes found in
`reftex-label-alist' and derive the correct label type in this way.
Possible values for this option are:

t This means to trust any label prefixes found.
regexp If a regexp, only prefixes matched by the regexp are trusted.
list List of accepted prefixes, as strings. The colon is part of
the prefix, e.g. ("fn:" "eqn:" "item:").
nil Never trust a label prefix.

The only disadvantage of using this feature is that the label context
displayed in the label selection buffer along with each label is
simply some text after the label definition. This is no problem if you
place labels keeping this in mind (e.g. *before* the equation, *at
the beginning* of a fig/tab caption ...). Anyway, it is probably best
to use the regexp or the list value types to fine-tune this feature.
For example, if your document contains thousands of footnotes with
labels fn:xxx, you may want to set this variable to the value "^fn:$" or
("fn:"). Then RefTeX will still do extensive parsing for any
non-footnote labels.

You can customize this variable.