Hi Gilbert, sorry for taking me so long to reply. To turn it off set permission_check_enabled_p to 0 (it's an openfts-driver parameter). You will realize a considerable improvement in performance.
The current scheme for checking permissions of the search results is inadequate IMO. I am probably going to make some changes in the next few days in order to have some items marked as public and items marked as private. The items will be marked as public or private w.r.t the result of the datasource operation (to be modified). In that case, we will only need to check permissions for private items (if permissions checking is on -- using the same parameter as above). I'm also planning to add an "indexing_p" field in the datasource operation so that you can have items of a content type that are not being indexed eventhough the content type is. For example, this is going to be useful when you only want to index moderated bboard postings.
I am probably going to change the way the openfts-driver does the permission checks. Here's what I have in mind (probably marking some items as private and others as public.