If you don't purged the reports from your database the lookup queries for the last report of a hosts start slowing down due to the way ActiveRecord queries for it:
Host.reports.last will generate the following SQL statement:

Report Load (1611.5ms) SELECT * FROM `reports` WHERE (`reports`.host_id = 19) ORDER BY reports.id DESC LIMIT 1

By changing that query to Host.find(host.id).reports.maximum('id')) we make a more efficient query improving the response.

Report Load (3.2ms) SELECT * FROM `reports` WHERE (`reports`.`id` = 85955)

Patch attached