138139 Execute 147 Creating sort index SELECT a.*, ug.show_title, s.locale, COALESCE(agl.setting_value, agpl.setting_value) AS author_g
This is related to the author index, which is legacy code and is known to perform poorly. (It will be removed entirely starting with 3.4.0.) See this related issue:
If you see a lot of requests for /search/authors (e.g. from search engines) in your access log around the time that performance degrades, it’s likely related to this. There is a suggestion here on how to quickly disable it.
Regards,
Alec Smecher
Public Knowledge Project Team