Inefficient notification-callback can block all available processes

Seems like a feature request. It works as designed.

The original goal was to prevent overwhelming Drupal by introducing a semaphore to hold off multiple simultaneous requests from Brightcove as even uploading a single video could cause desynchronization/inconsistency without a semaphore.
If I remember correctly the notification subscription support was introduced later in the module, and the mass create/delete was not considered as an issue at the time.

Although moving from away from semaphore to using queue to process the notifications could slow down the sync even more as every queue item needs to be processed in order to achieve full synchronization.

The queue system is already overwhelmed with the entity sync.