What exactly the flushadapterpacketqueue will do?
If there are packets queued for the particular network adapter they are removed from the queue and silently dropped.
Also at what frequency the packets will enter in queue means my packet processing is slow or what?
If the queue reaches its maximum size and causes newly coming packets to be dropped then probably your packet processing is slow. However, in reality packet queue size may vary depending of the system load.
Normally flushing packets is supposed to be used when you finish with packet processing and want to deallocate resources.