TagsController#index is terrible - response time goes up to 5 seconds.

Right now it has a response time of 4.49 seconds. Of which 3.82 are going into the view. But git timings are not better.

Steps to check metrics:

• go to http://performance.gitlab.net/dashboard/db/rails-controllers
• select the Action dropdown and search for TagsController#index
• check timings

Original discussion https://gitlab.com/gitlab-org/gitlab-ce/issues/12444