Investigate performance of Projects::MergeRequestsController#discussions

Reported as slow in #335329 (comment 620405174)

Dashboard: https://log.gprd.gitlab.net/goto/1228b988e958de29d5a90264dcf7bffb

json.meta.caller_id.keyword: Descending	Count	Operations over threshold
Projects::MergeRequestsController#discussions	4,674,131	121,601

Potentially related to

• #335236 (closed)
• #209784 (closed)
• #332967 (closed)