Slow new queries on ES 2.3.3. Free memory but high sdd reads

I have problem with very slow first queries (queries different than previously typed) on elasticsearch. Usually queries are fast but from time to time they can take up to 15s. The index size is 100GB on machine with 30GB of ram and I set ES_HEAP_SIZE=16GB. Elasticsearch version is 2.3.3. When the slow queries are executed the load on the hard drive is very high, where during the fast queries there is almost none. I use two fields for sorting the results so this might be doc_values which are read from hard drive.
And here comes the strange part. I have two almost exactly the same setups. The only difference is ubuntu version. On one it is 14.04 LTS (3.x kernel) and on the other one it is 16.02 LTS (4.x kernel). The later has this problem and the first one does not. There seems to be difference in memory usage. The later has around 10gb of free memory left and the first one around 200mb only. Is it possible that there are some kernel settings that play a role here? Maybe the memory is freed more aggresively? Or maybe should I look into something different?

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