Which columns to index with LEFT JOIN and GROUP_CONCAT
You have to create indices for all the fields you have used to bind tables or to restrict results:

wpp.ID 
object_id 
term_taxonomy_id 
wp_terms.term_id
wpp.post_author 
s.ID
post_type


Some of fields are already indexed as primary keys so do not duplicate indices, that can impact RAM consumption as well as performance.

First you have to do now - is to launch your query with EXPLAIN prefix and show the result.

N.B.

Always use aliases for joined tables, like third one in your code. Always specify explicitly table you refer to, like wp_terms.term_id or s.id.