I am testing kudu cluster performance these days, and found that the performance cannot grow up with machines added. With 1-master and 6-slaves, I got 3.5M TPS(350 million rows inserted per second), while when I add 3 more machines to the cluster, I only got 3.8M TPS. It seems the performance cannot grow up linearly with cluster extension. Why is that? Can anyone give some help? Thanks!