Slow and Freezes Often

First off, OS X Mountain Lion (and OS X Lion) is a little slower than OS X Leopard because it is more resource intensive than the latter. However, you can try a few things to see how to improve your speed in OS X Mountain Lion.

1. Repair Permissions - Go to Launchpad and type "Disk Utility". Open the application and in the sidebar click the Macintosh HD partition. Then click in the "First Aid" tab and click "Repair Permissions" wait for the process to finish and reboot your system. Test your system to see if it works better.
2. Boot in Safe Mode - Turn on your Mac and when you hear the boot chime, hit the "shift" key and hold it until the Apple Logo appears with a progress bar below. If you are not having any performance issues that could mean that a third party app or OS component could be the culprit. Safe Mode only boots the necessary operating system frameworks and restricts the ability to third party apps to boot automatically or call for OS services.
3. Check your Start Up items - Go to System Preferences> User & Groups. There click on the "Start Up Items" tab and see which applications are starting up when you boot your system. Disable them one by one until your issue is fixed.
4. Reset your PRAM and NVRAM
5. Reset your Mac's SMC
6. Download the OS X Mountain Lion 10.8.2 Combo Update

Hope this helps!