tuning feed item's table query with multiple condition
I dont know about feed_id but your timestamp column by definition have probably many differents values. I will advise you to try to create a simple index on timestamp column. 

Do you have "disparate" values too on feed_id column?

For me you should take your attention on the timestamp column (WHERE + ORDER BY). Maybe it'll not a index problem, maybe your MySQL instance have some difficulties to sort the result (bad temp table management) so you can try without ORDER BY clause to see its impact.

Let me know.

Max.