Index on cp_ad_pop_total recommendation
We have heavily modified Classipress so it could very well be that this is not 100% necessary, but we found when sorting by popular ads, this is part of the query:
LEFT JOIN wp_cp_ad_pop_total a ON (wp_posts.ID = a.postnum AND a.postcount > 0)
This can be a pretty harsh condition on a site with many ads (over 3,000). We had some of these queries taking up to 25 seconds to complete. By adding an index for the postnum column, it reduced it to well under a second.
Just a suggestion.