performance issues on indexed keys

> db.SondeDataContainer.stats()
{
	"ns" : "netbeams.SondeDataContainer",
	"count" : 2419200,
	"size" : 1170892976,
	"storageSize" : 1358632416,
	"nindexes" : 20,
	"ok" : 1
}

All documents have the following structure

> db.SondeDataContainer.findOne()
{
	"_id" : ObjectId("046f4007e705124bbedf9200"),
	"message_id" : "49867084-9f47-4412-bff0-a5c517bc6c7a",
	"sensor" : {
		"ip_address" : "192.168.0.123",
		"location" : {
			"latitude" : 37.89155,
			"longitude" : -122.4464
		}
	},
	"time" : {
		"valid" : "Sun Nov 22 2009 21:45:06 GMT-0800 (PST)",
		"transaction" : "Sat Nov 28 2009 21:25:58 GMT-0800 (PST)"
	},
	"observation" : {
		"WaterTemperature" : 46.47,
		"SpecificConductivity" : 44.7,
		"Conductivity" : 110.1,
		"Resistivity" : 5362.29,
		"Salinity" : 0.01,
		"Pressure" : 0.62,
		"Depth" : 2.075,
		"pH" : 7.64,
		"pHmV" : -50.7,
		"Turbidity" : 0.3,
		"ODOSaturation" : 105.9,
		"ODO" : 32.16,
		"Battery" : 4.4
	}
}

The Indexes were created for all the keys... Here's the performance problem to 
search 2 different keys with different values... The search for the ip address 
is fast... (there are only 256 different ip address 192.168.0.x, 1<x<256), The
 problem is when I search over the observations keys, which have all random 
values...

> db.SondeDataContainer.find( {"sensor.ip_address":"192.168.0.102"} ).explain()
{
	"cursor" : "BtreeCursor sensor.ip_address_",
	"startKey" : {
		"sensor.ip_address" : "192.168.0.102"
	},
	"endKey" : {
		"sensor.ip_address" : "192.168.0.102"
	},
	"nscanned" : 9505,
	"n" : 9505,
	"millis" : 111,
	"oldPlan" : {
		"cursor" : "BtreeCursor sensor.ip_address_",
		"startKey" : {
			"sensor.ip_address" : "192.168.0.102"
		},
		"endKey" : {
			"sensor.ip_address" : "192.168.0.102"
		}
	},
	"allPlans" : [
		{
			"cursor" : "BtreeCursor sensor.ip_address_",
			"startKey" : {
				"sensor.ip_address" : "192.168.0.102"
			},
			"endKey" : {
				"sensor.ip_address" : "192.168.0.102"
			}
		}
	]
}

> db.SondeDataContainer.find( {"observation.pH":4.52, "observation.Battery":1.4} ).explain() 
{
	"cursor" : "BasicCursor",
	"startKey" : {
		
	},
	"endKey" : {
		
	},
	"nscanned" : 2419200,
	"n" : 28,
	"millis" : 53116,
	"oldPlan" : {
		"cursor" : "BasicCursor",
		"startKey" : {
			
		},
		"endKey" : {
			
		}
	},
	"allPlans" : [
		{
			"cursor" : "BasicCursor",
			"startKey" : {
				
			},
			"endKey" : {
				
			}
		}
	]
}

What's the problem?