MongoDB（java）需要很长时间才能找到

Question

我有一个mongdb数据库，其中包含以下统计信息：

{
        "ns" : "pourmoi.featurecount",
        "count" : 12152142,
        "size" : 1361391264,
        "avgObjSize" : 112,
        "numExtents" : 19,
        "storageSize" : 1580150784,
        "lastExtentSize" : 415174656,
        "paddingFactor" : 1,
        "paddingFactorNote" : "paddingFactor is unused and unmaintained in 3.0.
It remains hard coded to 1.0 for compatibility only.",
        "userFlags" : 1,
        "capped" : false,
        "nindexes" : 2,
        "totalIndexSize" : 1165210816,
        "indexSizes" : {
                "_id_" : 690111632,
                "feature_1" : 475099184
        },
        "ok" : 1
}

和一个执行find并返回大约50个结果的java程序。

这是查询

db.featurecount.find(
{ "$or" : [ { "feature" : "hello"}, { "feature" : "how"},
            { "feature" : "are"} , { "feature" : "you"} ]})
.sort({count: -1}).limit(20)

此查询大约需要30秒（至少）...是否可以让它运行得更快？

PS：mongodb服务器在localhost ...