MongoDB too many records?
===

I have a PHP app that interact with MongoDB. Until recently, the app was working fine but a few days back I found that the app is starting to respond REALLY slow. One of the collections has shot up to 500K+ records. So the MongCursor for any query on that collection keeps timing out. 

I don't think 500K records is WAY too much. Other pages using mongodb are beginning to slow down as well, but not as much as the one which uses the collection with 500k records. Static pages which don't interact with MongoDB are still fast to respond. 

I am not sure what could be the issue here. I have indexed the collections, so that does not seem to be a problem. Another point to note is that the RAM spec on the server is 512 MB and when PHP executes Mongo, top command show 15000k memory free. 

Any help will be greatly appreciated. 