Continue to improve performance of Advanced Search API

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This issues is created as a follow-up to &3166 (closed). While significant improvements have been made in terms of response time of Advanced Search API, the latency is still high in our current 10k reference architecture. According to our performance targets, I am creating this ~S2 issue to track the future improvements.

Note, the Postgres instance high CPU was discussed &3166 (comment 377220591), and it's verified that the performance issues can be resolved by increasing the number of (v)CPUs of Postgres instance. Please see gitlab-org/quality/performance#293 (closed) for the details.