Elasticsearch indexing is very slow

I have a Titan database with Cassandra storage backend, and I am trying to create a mixed index based on two property keys.

I am able to register the Index using following commands:

graph=TitanFactory.open(config);
graph.tx().rollback()  

m = graph.openManagement();
m.buildIndex("titleBodyMixed", Vertex.class).addKey(m.getPropertyKey("title")).addKey(m.getPropertyKey("body")).buildMixedIndex("search");
m.commit();

m.awaitGraphIndexStatus(graph, 'titleBodyMixed').status(SchemaStatus.REGISTERED).timeout(3, java.time.temporal.ChronoUnit.MINUTES).call();

And when I am checking, the Index is successfully registered after a few seconds. At next step, I try to reindex the database using the following commands:

m = graph.openManagement();
m.updateIndex(m.getGraphIndex('titleBodyMixed'), SchemaAction.REINDEX).get();

However, updateIndex command is not finishing, (After 12 hours).

I have about 300k data entry in the database and each data entry has one Title and one Body to index.

My question is that how can I speed up the indexing?

When I am using top command I see that my CPU is not saturated by indexing processes:

My Titan config file is as bellow:

config =new BaseConfiguration();
config.setProperty("storage.backend","cassandra");
config.setProperty("storage.hostname", "127.0.0.1");
config.setProperty("storage.cassandra.keyspace", "smartgraph");
config.setProperty("index.search.elasticsearch.interface", "NODE");
config.setProperty("index.search.backend", "elasticsearch");

The following is showing elasticsearch service properties:

curl -X GET 'http://localhost:9200'
{
  "status" : 200,
  "name" : "Ms. Marvel",
  "cluster_name" : "elasticsearch",
  "version" : {
    "number" : "1.7.2",
    "build_hash" : "e43676b1385b8125d647f593f7202acbd816e8ec",
    "build_timestamp" : "2015-09-14T09:49:53Z",
    "build_snapshot" : false,
    "lucene_version" : "4.10.4"
  },
  "tagline" : "You Know, for Search"
}