@isotopp Thanks for reaching out! It's MySQL 5.7, running as a service so getting access to the box is a little more difficult.
Slow queries are on now, but logging to a file I can't figure out how to get access to unfortunately.
I made a little progress and discussed the issue a little more here -- would be curious if you have any other input, or if you think partitioning the table might be worth it: https://github.com/writefreely/writefreely/issues/741