Just finished debugging a problem where I had text boxes whose performance was terrible. There was a noticeable lag between input and display and it was not usable.
After looking around it turned out the CSS3 box-shadow I had on the parent div which contained these text boxes was causing the performance hit. I assume that each time you type a letter, the browser has to re-render the box-shadow. I was using a box shadow of type inset for an inner gradient look so your results may differ with different box shadow types.