That is strange, but I assume that there are should be very simple explanation, considering so much difference (8-10x times).

Have you tried to run the queries with profiling enabled https://www.elastic.co/guide/en/elasticsearch/reference/current/search-profile.html, I believe it is a new API in ES 2, so you will not be able to compare results with elastic search 1.7, but maybe it can get better picture what can slow down the queries in 5.x.

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