It's been a long day, but I've been wresting with a query which wont use an index, and thus takes much longer than it should. I stumbled upon
USE INDEX FOR JOIN, but it didn't improve anything.
Then I reverted it back to the original query.
Now it's decided to use that index, instead of the previous full table scan and it's down from 17s to 1s.
Did I "convince" the query planner to use the index with
USE INDEX FOR JOIN?