CPU intensive /api/v4/projects/id/merge_requests

Following the https://gitlab.com/gitlab-org/gitlab-ce/issues/62876#note_190720305

We discovered that the /api/v4/projects/{{project_id}}/merge_requests is very CPU-intensive tasks that increases latency when running on Puma.

We should investigate what is wrong with this endpoint and try to understand what it takes to optimize it.

For our testing, we should use https://gitlab.com/gitlab-org/quality/performance/#2-data-for-load-tests.

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/64335 and gitlab-org/quality/performance#40 (closed).