Sorry I upgraded to Mountain Lion

I upgraded to Mountain Lion on 7/28 & have now found my computer has slowed down significantly. I have a less than two month old iMac 21.5 with 12 GB RAM. Prior to upgrading, this computer was very fast in everything it did. Now I sit & wait for everything it does. Now it runs like my computer did when it only had 4 GB RAM & yes it still recognizes it's running with the 12 GB RAM. Wishing I could go back to 10.7 This thing runs like a slow running PC.