Capped Collection in mongodb issues

Question

I made an analysis on capped collection, I found that there is no performance improvement in capped collection. I created a collection named test1 with 20,000 data, I did copyTo test2 with same data which is capped true with data size specified. I gave the following queries to examine the performance

 db.test1.find( { $query: { "group" : "amazonTigers"}, $explain: 1 } ).pretty()


 db.test2.find( { $query: { "group" : "amazonTigers"}, $explain: 1 } ).pretty()

 Both result in the same response time as 124ms...

1. More Over, I dont understand how the capped collection works???

2. I read through lots of blogs, But am not able to find the correct working principle of mongo capped collection.

3. I read through the disadvantages of capped collection, Its given like we are not able to use $set and $push in it. Is there any other disadvantages there in capped collection for the specified collection entries???

Regards, Harry