We have a 8 node cluster running in DRAM+DISK mode (I2.xlarge) instances .
Was giving good performance with 99% calls within 1ms
Yesterday we replaced 1 node with I3.xlarge and cpu wait time was even less on the new box.
Today we replaced two more boxes with I3.xlarge and since then we have seen a degradation in write performance on the all nodes (old and new). 50% writes have gone beyond 1ms upto 4-8 ms bucket