SQL Server 2012 Explicit transactions taking time with Always on Setup
Got all the answers to my question mentioned above. If anyone facing the same problem I would like to point them to the answer :
Here's the answer to all the questions why explicit commit takes time :

[A commit transaction (FlushToLSN/StartLogFlush) can trigger similar behavior, even when the block is not full, so a commit transaction does not have to wait for the block to become full.]

Please find more details on following link : 
https://support.microsoft.com/en-gb/kb/230785]

Thank you.