最近在测试 mongodb 的 2dsphere 索引，在 900w 数据情况下发现速度慢的可怕，求指导啊？

文档内容很简单，就几个字段当然主要的是坐标ugps字段已经建过2dsphere索引
db.client_request_real.ensureIndex({"ugps":"2dsphere"},{"name":"_ugps_", "background" : true })
然后在指定的一个4变形区内搜索
var wxgd ={
"type":"Polygon",
"coordinates":[[
[120.2673590, 31.5510940],
[120.2673590, 31.5710940],
[120.2873590, 31.5710940],
[120.2873590, 31.5510940],
[120.2673590, 31.5510940]
]]}
db.client_request_real.find({"ugps":{"$within":{"$geometry":wxgd}}}).sort({"time":-1}).limit(20)

explain显示
> db.client_request_real.find({"ugps":{"$within":{"$geometry":wxgd}}}).sort({"time":-1}).limit(20).explain()
{
"cursor" : "S2Cursor",
"isMultiKey" : true,
"n" : 20,
"nscannedObjects" : 33446,
"nscanned" : 447373,
"nscannedObjectsAllPlans" : 33446,
"nscannedAllPlans" : 447373,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 7,
"nChunkSkips" : 0,
"millis" : 35337,
"indexBounds" : {

},
"nscanned" : 447373,
"matchTested" : NumberLong(413927),
"geoTested" : NumberLong(413927),
"cellsInCover" : NumberLong(2),
"server" : "syslog1:27017"
}
>
狂汗这个速度好像也太慢了，查询期间用dsata查看了下系统负载
cpu、磁盘、内存均无压力。这是咋回事啊？

文档示例数据：
{ "_id" : ObjectId("531ebe0ea4e3a228463350a6"), "v" : "2", "key" : 0, "routeid" : 359, "segmentid" : "34436760", "stationseq" : 3, "ugps" : { "type" : "Point", "coordinates" : [ 120.275593, 31.567584 ] }, "ugps_type" : 0, "time" : ISODate("2014-03-11T07:40:38Z") }