Hi Mahendra,
Given that you've created the Task queue with more than 1 thread, perhaps it has to do with the available number of cores? See Thread count recomendation.
From the documentation:
"Microflows or Java actions that are run in the background run as soon as possible in the order they were created, but possibly in parallel. They are consumed in FIFO order, but then run in parallel in case of multiple threads. (...)"
Hope this helps,
João