it sounds like an expensive query. anyway, make sure you have the proper indexes on tables

i was trying, but i can't make an index on multiple tables, so i can't remove the filesort there

it's about finding topics (+eagerloading of the last post, the last post author and the forum) where a post from a certain user exists

ordered by the date of the last post

sometimes it helps to go for de-normalization or create new tables that are optimized for certain queries