Slow (high latency) upon queries with sort mongoDB - codeigniter

Question

I am currently migrating from couchdb to mongodb, still learning this stuff and have a problem anytime I do some queries with sorting in my webpage.
I am using codeigniter as the framework and using alexbilbie lib for mongodb php library.
So, here is my problem:
I intend to do queries from sensors in a room that updated each second (thus, already saved thousands docs in collection) and to get each latest sensor value I use this query from model:
function mongoGetDocLatestDoc($sensorid){
$doc = $this->mongo_db->where(array('SensorId'=>$sensorid))->limit(1)->order_by(array('_id'=>'DESC'))->get('my_mongo');
return $doc;
}
if I called this with my controller, it took a lot of time to process the query and even worse if I change the sort by timestamp. and it is double the latency each time I called this again for the second sensor, let alone I have more than 10 sensors that need this query in the same page. Am I doing it wrong or there is some more efficient way to get the latest data from collection?
edit:
#Sammaye : I tried making an index based on your suggestion and here is the explain generated after I executed the query:
"cursor" : "BtreeCursor timestamp_desc",
"nscanned" : 326678,
"nscannedObjects" : 326678,
"n" : 50,
"millis" : 4402,
"nYields" : 7,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
"timestamp" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
]
}
as per comparison, this explain the first query without using index (that executed faster) :
"cursor" : "BasicCursor",
"nscanned" : 385517,
"nscannedObjects" : 385517,
"n" : 50,
"millis" : 1138,
"nYields" : 0,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
}

Answer 1

Okay thank you all for your great response :) Here is my finding after trying with indexes:
Update the mongodb to the latest version. I found this slightly improve my query. I am updating from default 2.0.3 that provided as default in ubuntu 12.04.02 to mongodb version 2.4.3
Build indexes/compound indexes based exactly on how you mostly needed in your query. as an example, in my question, my query based on SensorId and _id: DESCENDING so the best strategy to optimize my query would be something like :
db.your_collection.ensureIndex({ "SourceId" : 1, "_id" : -1 },{ "name" : "sourceid_idx", "background" : true });
or in other case if I needed it based on timestamp:
db.your_collection.ensureIndex({ "SourceId" : 1, "timestamp" : -1 },{ "name" : "source_idx", "background" : true });
I found a very good explanation about mongodb indexes here
Hope this will help other people that stumbled upon similar problem...