Spike: identify opportunities to improve top-level group MR list endpoint performance

Context

This is the first step in kicking off gitlab-org#20250.

Identify opportunities to improve the performance of this worker. This will serve as the basis to create an implementation plan.

This is a spike and up to the assignee to define the approach. Some ideas:

Inspect latest performance data to detect patterns.
Understand what these customers (the top outliers causing most apdex problems) are trying to achieve.
Add timing logging to confirm where the bottlenecks are.
Connect with experts for any specialized input needed (e.g. database).
Identify if different API restrictions should be put in place.
- Example: #585176
Consult with devopsplan on their traversal ID effort, which solves a similar problem for work items.
- https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/traversal_ids_on_issues/
- gitlab-org#18644 (closed)
...