When I try to highlight a word, the little definition box comes up and goes away too fast.

If I try to highlight & see the defintion a word (it doesn't matter if I highlight & press 3 fingers on my trackpad, or just right click and press "Look Up". What occurs is either the definition box goes away within half a second or doesn't show up at all. My mac has also been severly slower ever since I upgraded to mavericks. 😟