Remote Publishing

I am using xmlrpc api for remote publishing. In starting when my website has few posts, I was able to publish upto 10 to 11 posts per minute. But as the number of post increases in my website this number drops down to 3 to 6 post per minute (presently my website has 150k posts and was able to post 4 posts per minute).
Website is self hosted on Amazon ec2 instance(4 core, 15GB ram).
What are the possible solutions to increase the per minute publishing rate.