By default Tigase pubsub component caches node configuration of 2000 last loaded nodes. If there are many requests to database to load node configuration and your installation contains many nodes it may be a good idea to increase number of cached nodes.
To do this you need to set pubsub-repository-cache-size property of pubsub component to new size.
pubsub () {
pubsub-repository-cache-size = 4000
}