On our ES 6.x (various versions tested with 6.4.1 being the last one) installed on Ubuntu 16 hosts we are experiencing temporary problems with Indexing queries performance.
Single query can take up to 10 or more seconds to return results to client (we use curl for testing).
But if we see into elasticsearch logs and even find the specific query there it shows normal time (usually couple of milliseconds).
We didn't notice such problems with GET queries. Only Index.
Does anybody have any idea what can cause the difference between the time in ES log and the time it takes the whole process to return query result to client?
We were able to eliminate network problems between client and server machine as the situation takes place even when we query ES from localhost.