The bulk API becomes much slower once I have indexed 300,000 documents into elasticsearch?

Elastic Stack Elasticsearch
7

Here is the logs of the mongo-connector.

2015-09-26 08:33:28,648 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.105s]
2015-09-26 08:33:31,011 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.134s]
2015-09-26 08:33:32,265 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.129s]

    the above is the 600th bulk request, it takes around 1s : 08:33:32,265 - 08:33:31,011 
    the next is the 601th bulk request, it takes around 10s:  08:33:43,476 - 08:33:32,265

2015-09-26 08:33:43,476 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.571s]
2015-09-26 08:33:54,070 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.568s]
2015-09-26 08:34:03,602 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.569s]
2015-09-26 08:34:13,100 [INFO] elasticsearch:64 - POST http://172.31.1.254:9200/_bulk?timeout=60&refresh=false [status:200 request:0.570s]