How can I go back to Mountain Lion from Mavericks
Looking on google would have avoid this question. As Macworld UK described it, here are the steps : 


Get a copy of Mountain Lion
Download the recovery Disk Assistant
Create a recovery install Disk
Restart in Recovery Mode
Erase your Main HD. 
Install a fresh OS. 




BUT : 

IMHO, your computer should be fast enough to handle Mavericks. The problem isn't probably the system itself. You should look at the Activity Monitor to see if there are some anomalies (high memory pressure, high CPU% etc.)