Btw this documenting nicely what happens, merging is falling behind exactly for some time, then it all performs well :
15:05:25,915][INFO ][indices.store ] [es-884a70a1805147db95b8a28e776cd59a] updating indices.store.throttle.type from [merge] to [none]
15:05:25,966][INFO ][index.shard ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][4] updating refresh_interval from [1s] to [-1]
15:05:25,971][INFO ][index.shard ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][0] updating refresh_interval from [1s] to [-1]
15:05:25,974][INFO ][index.shard ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][2] updating refresh_interval from [1s] to [-1]
15:05:25,974][INFO ][index.shard ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][3] updating refresh_interval from [1s] to [-1]
15:05:25,974][INFO ][index.shard ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][1] updating refresh_interval from [1s] to [-1]
15:05:26,731][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][4] now throttling indexing: numMergesInFlight=4, maxNumMerges=3
15:05:26,807][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][0] now throttling indexing: numMergesInFlight=4, maxNumMerges=3
15:05:26,865][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][2] now throttling indexing: numMergesInFlight=4, maxNumMerges=3
15:05:26,922][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][4] stop throttling indexing: numMergesInFlight=2, maxNumMerges=3
15:05:26,979][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][3] now throttling indexing: numMergesInFlight=4, maxNumMerges=3
15:05:27,018][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][1] now throttling indexing: numMergesInFlight=4, maxNumMerges=3
15:05:27,060][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][2] stop throttling indexing: numMergesInFlight=2, maxNumMerges=3
15:05:27,067][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][0] stop throttling indexing: numMergesInFlight=2, maxNumMerges=3
15:05:27,139][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][3] stop throttling indexing: numMergesInFlight=2, maxNumMerges=3
15:05:27,330][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][1] stop throttling indexing: numMergesInFlight=2, maxNumMerges=3
15:05:29,024][INFO ][index.engine ] [es-884a70a1805147db95b8a28e776cd59a] [mi_2015_10_w4][3] now throttling indexing: numMergesInFlight=4, maxNumMerges=3