Problems with Display from OS X 10.9 on 27 inch mac

Since I downloaded the OS X 10.9, the Display is either too small to read, or when I make it larger, the entire information moves to the right, and is off the edge, not to be seen. Not only that, but my computer has slowed down considerably since I downloaded that. I feel that Apple is punishing me for purchasing the 27 inch mac. How can I fix the Display so I (a.) can read it, and (b.) can read the entire page? How can I fix the slow function of this OS X 10.9?