Q:

mongodb got slow when the document count went around 100, 000 . Any performance optimization?

I run a single mongodb instance which is getting inserted with logs from an app server. the current rate of insert in production is 10 inserts per second. And its a capped collection. i DONT USE ANY INDEXES . Queries were running faster when there were small number of records. only one collection has that amount of data. even querying from collection that has very few rows has become very slow. IS there any means to improve the performance.
-Avinash

A:

This is a very difficult question to answer because we dont know much about your configuration or your document structure.
One thing that immediately pops into my head is that you are running out of memory.  10 inserts per second doesn't mean much because we do not know how big the inserted documents are.
If you are inserting larger documents at 10 per second, you could be eating up memory, causing the operating system to push some of your records to disk.  
When you query without using an index, you are forced to scan every document.  If your documents have been pushed to disk by the OS, you will begin having page faults.  Mongo will need to fetch pages of data off the hard disk, and load them into memory so that they can be scanned.  Before doing this, the operating system will need to make room for that data in memory by flushing other parts of memory out to disk.
It sounds like you are are I/O bound and the two biggest things you can do to fix this are

Add more memory to the machine running mongod
Start using indexes so that the database does not need to do full collection scans

