Fetching All Posts (200+) in a Category – Performance Issues?

If you want to query for a large set of posts, but don’t need full information for each you can use 'fields' => 'ids'm see Return Fields Parameter.

In this case, I concur with comment, it might make more sense to simply retrieve adjacent post several times. You’ll have to use setup_postdata() and wp_reset_postdata() since get_adjacent_post() doesn’t allow you to set post explicitly. Unfortunately that function isn’t too friendly to reuse or make use of SQL it generates.