the shape doesn't really matter as much here, but it would be nice to initialize it in the constructor and have a little comment about when it would be flagged/fired.

it seems like the comment might be something like:
The perf event should be sent when the first splice is generated, which should be triggered by the cached data for the inbox.  If the cached data is not available, the event will fire as soon as any messages are provided to us by the back-end as a result of synchronization for initial sync, or when the batch load happens if we are priming the cache because this was an upgrade.