We've (in place) upgraded from 2.4 to 5.1.2 and since we get sporadic slowdowns with simple document GET queries.
Aggregating a specific query type for the same day with the old 2.4 cluster and the upgraded 5.1.2 cluster yields the following numbers:
ES5: N Min Max Median Avg Stddev x 80801 0.433811 28338.847 1.481119 321.63365 1296.8558 ES2: N Min Max Median Avg Stddev x 78746 0.479862 383.48786 1.411027 2.259211 6.7429293
ES2 has an average response time of 2.25 ms with a max of 383 ms, while ES5 has an average of 312 ms and a max of 28.3 seconds...
What's strange the median looks OK, but there are some very slow queries.
Apart from the 2.4->5.1.2 server (and 2.x->5.x java client) upgrade , nothing has changed in the cluster.