MQTT (and not only) is severely effected on its performance while subscribing/unsubscribing when many topics are existing because:

• SimpleAddressManager::getMatchingBindings/getDirectBindings are performing a linear search on all the existing Bindings
• the results of the previous matches are saved into a Bindings instance even if not necessary (note: Bindings perform atomic operations while saving the matching Binding)
• All the MQTT operations are performed in the Netty event loop