How to optimize magento performance?

You need to follow below points:

1) Disable any unused modules
2) Enable magento caching
3) Disable magento log
4) Combine external js/CSS in one file
5) MySql query caching
6) Image optimization
7) Enable Gzip compression

What is difference between Mage::getSingleton() and Mage::getModel()

The main difference is that Mage::getSingleton() does not create new object if the object for same class is already created.

But Mage::getModel() creates new objects for the class everytime when it is called.