I love the concept of this plugin. Very useful even with this performance flaw. But would like to point it out for a more long term fix.
It seems as though two query calls are made per series posts when pulling up a page which displays the series list/titles. So, if you have 20 titles/posts in a series, that means 40 queries.
Heres an example of one set for a post:
SELECT post_id, meta_key, meta_value FROM wp_postmeta WHERE post_id IN (493)
SELECT t.*, tt.* FROM wp_terms AS t INNER JOIN wp_term_taxonomy AS tt ON tt.term_id = t.term_id INNER JOIN wp_term_relationships AS tr ON tr.term_taxonomy_id = tt.term_taxonomy_id WHERE tt.taxonomy IN ('category') AND tr.object_id IN (493) ORDER BY t.name ASC
The reasoning for this seems to be in the required replace for the tag: %post_title_list%
The function get_series_posts is being called for each posts. This is happening around line 300 in series-utility.php
My work around for now was simply to create my own function with my own styling built into it. Thus avoiding the need for tags and more than 2 queries for any series list display.
I imagine others will see the same performance as their series lists grow. Perhaps this will be fixed at a later date?
Thanks much for your time on this plugin!