Slow frontend rendering of merge request

This might be related to https://gitlab.com/gitlab-org/gitlab-ce/issues/49059, but @sytses mentioned that he's sees a lot of slowness in: https://dashboards.gitlab.net/d/1EBTz3Dmz/sitespeed-page-summary?orgId=1&var-base=sitespeed_io&var-path=default&var-group=gitlab_com&var-page=_gitlab-org_gitlab-ce_merge_requests_9546&var-browser=chrome&var-connectivity=native&var-function=median

That link attempts to load https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9546. The Lighthouse report shows:

One peculiar thing here is that I see two codeclimate.json files being loaded (https://gitlab.com/gitlab-org/gitlab-ee/issues/2737#note_68595587), and they take a long time:

There are also a lot of DOM nodes:

Full report:

gitlab.com_2018-09-09_21-23-31.json

/cc: @timzallmann, @leipert