Rails action using an insane amount of MySQL/CPU

The problem is that paginate uses :limit and :offset which can't work
well with eager joins (your :include => 'tags') since all the matching
rows (perhaps thousands or millions) have to be pulled into Ruby,
parsed, then limited and offset. To put it mildly, this does not

Remove the :include => tags to regain nearly all your performance at
the minor cost of 12 additional queries to pull tags per page.

Generally speaking, to troubleshoot database issues, look at the slow
queries in your production.log and use EXPLAIN <the query> in MySQL to
see why they're performing poorly. Luckily, in this case, it's just a
matter of returning way too much data.