Limit search to page/post title only

Yes, that would be more efficient. You can turn off everything else from Relevanssi indexing settings, but to disable content indexing, you need this:

add_filter( 'relevanssi_index_content', '__return_false' );

Works perfectly! Thank you very much.

(In this specific case, the index was reduced from 72977 to 2479 terms without the search losing any of its usefulness.)