Improve performance of users API under load

The TTFB P90 for the users API is taking longer then the expected 200ms

NAME         | RPS    | RPS RESULT           | TTFB AVG  | TTFB P90           | REQ STATUS     | RESULT
-------------|--------|----------------------|-----------|--------------------|----------------|-----------------
api_v4_users | 1000/s | 500.46/s (>800.00/s) | 1860.73ms | 4331.20ms (<200ms) | 100.00% (>99%) | FAILED²

This is based on the time to retrieve a full page of 20 users, when changing the pagination to 100 users per page the time increases.

NAME         | RPS    | RPS RESULT           | TTFB AVG  | TTFB P90            | REQ STATUS     | RESULT
-------------|--------|----------------------|-----------|---------------------|----------------|-----------------
api_v4_users | 1000/s | 108.97/s (>800.00/s) | 8317.08ms | 11324.43ms (<200ms) | 100.00% (>99%) | FAILED²

This was tested against a 50k Reference architecture. More information on the tests results with different user amounts can be found in gitlab-org/quality/performance#487 (closed).

As per our performance targets this endpoint's TTFB metric is above the target of X ms which is severity2. Task is to improve the endpoint's performance into next tier.

Steps to reproduce

Check out the GitLab Performance Tool
Run the specific test with the run-k6 command. For example against the 10k environment you would run this following from the project root: ./run-k6 -e environments/50k.json -o 60s_1000rps.json -t api_v4_users.js.
- You will need an ACCESS_TOKEN for this endpoint as well.
- Ensure you have more than 7 users to start seeing failures.
If you're seeking to run the test against your own environment the Tool's documentation has details on how to achieve this.