MR page rendering slower than usual when it has a lot of discussions

On top of addressing performance optimisations when posting new notes on MRs with a lot of discussions for 11.6, we'd like to better understand what might be causing issues sparking different reports of the MR page being rendered slower than usual.

1

Checking our dashboard for historical performance on a MR with substantial size, we can see spikes for last visual change in recent daysScreen_Shot_2018-11-09_at_14.49.12

(^ this is the MR measured in the above dashboard: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9546)

2

In this otherMR: gitlab-ui!63 (merged) discussions.json is taking around ~20s to load

Screen_Shot_2018-11-09_at_14.51.40

More info on historical performance of this endpoint (thanks for the heads up @smcgivern): https://dashboards.gitlab.net/d/thYzurImk/rails-controllers?orgId=1&from=now-7d&to=now&var-action=Projects::MergeRequestsController%23discussions.json&var-database=influxdb-01-inf-gprd

Edited by André Luís