I use single java client to do range query with two constraints and i have create index on query key, the read speed only can reach to 6M/s (Read back 6M, nearly spend 1.1 second ), but my network card is "1000Mbit/s" and all of query data have been in memory(The data size < Memory size)?

stats info:

"objects" : 2936299,
"avgObjSize" : 357.40783210429186,
"dataSize" : 1049456260,
"storageSize" : 1181462784,
"indexSize" : 370705344,
"fileSize" : 4226809856,

select report_path from table where visit_time > start_visit_time and visit_time < end_visit_time
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BasicDBObject range = new BasicDBObject();
range.put("$gt", start_visit_time);
range.put("$lt", end_visit_time);

BasicDBObject query = new BasicDBObject();
query.put("visit_time", range);

BasicDBObject keys = new BasicDBObject();
keys.put("report_path", 1);
keys.put("_id", 0);

DBCursor cur = ct.find(query, keys);
while (cur.hasNext())