We have scenarios which have high value of the queues that fill the broker space.Can you please let me know , how to increase the velocity of subscription and how to reduce the filling of queues in the broker.

check if the trigger which is subscribing the document is serial or concurrent.
If serial then change it to concurrent so that the processing will be fast.

Next incraese the “Maximum Threads” so that the number of threads available for triggers will be sufficient and the processing will be fast.

Next it depends again what you are doing with this document suppose you are picking this doc from broker queue and directly inserting into a DB and if the DB is slow processing the transactions your overall processing will become slow. Need to check whats happening here.

To reduce the filling on queue in broker you need to check the publishing type (i.e how the documents are created & published to broker) based on that we can think of some way to limit the queue filling in broker.