Related to #11583 -- currently when we generate a sync response we cache it for a period of time (as clients sometimes timeout and retry). Unfortunately the bundled aggregations are added during the serialization step, which occurs after caching, and is re-done even when using a cached response.

Ideally we should be calculating the bundled aggregations earlier in a different manner and the serialization methods should be synchronous.

See https://matrix.to/#/!XaqDhxuTIlvldquJaV:matrix.org/$1E_K64Dd5p6iFMS4agak62bn5JZl3Ry-6XiTXYzOmhU?via=matrix.org&via=vector.modular.im&via=envs.net for a brief conversation around this.