## Problem
After 2.13, server CPU stepped up and stayed up. Sentry profiling of
`POST /metadata` pins it on
`addFlatEntityToFlatEntityMapsThroughMutationOrThrow` — ~26% self-time
plus a long tail, turning metadata-write requests into multi-second
(~18s observed) operations.
The hot stack is:
```
WorkspaceMigrationValidateBuildAndRunService.computeAllRelatedFlatEntityMaps
└ getSubFlatEntityMapsByApplicationIdsOrThrow
└ addFlatEntityToFlatEntityMapsThroughMutationOrThrow
```
Every metadata migration rebuilds the twenty-standard application's flat
sub-maps — thousands of entities, across every involved metadata type —
through this util.
## Root cause
`addFlatEntityToFlatEntityMapsThroughMutationOrThrow` maintains
`universalIdentifiersByApplicationId` and deduped each insert with
`Array.includes`:
```ts
if (!existingUniversalIdentifiers.includes(flatEntity.universalIdentifier)) {
existingUniversalIdentifiers.push(flatEntity.universalIdentifier);
}
```
That scan is O(n) per insert, so building a map for an application with
`n` entities is **O(n²)**. The twenty-standard application groups
thousands of standard entities under one `applicationId`, so its sub-map
rebuild dominates.
The dedup is also redundant: the function throws `ENTITY_ALREADY_EXISTS`
at the top if the `universalIdentifier` is already in
`byUniversalIdentifier`, and every id pushed to the per-application list
is also written there. So reaching the push guarantees the id is new —
the `.includes()` is always `false`.
## Fix
Drop the scan and push directly → map building is **O(n)**. Behavior is
unchanged (the early throw already enforces uniqueness).
## Test
Adds a unit spec covering indexing, the no-`applicationId` case, the
duplicate throw, and a 20k-entity build that completes instantly (guards
against re-introducing the quadratic).
## Follow-up (separate PR)
This is the bleed-stopper. The deeper issue is that
`computeAllInvolvedApplicationIds` pulls the **entire** twenty-standard
application into the dependency set of every migration and rebuilds
those sub-maps per request instead of caching them. Worth scoping the
dependency set to referenced entities (or caching the standard-app
sub-maps), which I'll raise separately.
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Félix Malfait
02d6e2d76f
