feature-benchmark: Try measuring from earlier in InitialLoad scenarios

[def-]

Doesn't help:

	PgCdcInitialLoad                    | wallclock       |           4.474 |           2.576 |   s    |    10%     |    !!YES!!    | worse:  73.7% slower

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.