Improve Placeholder readiness time for Flex

#### What problem would the feature you're requesting solve? Please describe.
Currently it takes ~8-10 seconds on P50 for Flex placeholders to come up and be ready for specialization.

This has direct impact on COGS and placeholder capacity planning. We need to understand where the majority of time is spent and how we can improve it.

There is a thread with lots of details and queries....

Start by locating the referenced thread and reviewing its details and queries about Flex placeholder startup. Measure where the roughly 8–10 second P50 readiness time is spent, then define and validate an improvement that reduces readiness time while preserving specialization readiness and capacity-planning expectations.

Written by the indexing model from the issue text.