Optimise JavaScript DOM update
First off, your code doesn't seem very optimal. This doesn't effect the speed as far as I can see, but does effect the readability, which may limit the help you get here.


Why do you create a second div inside #grid?
Why do you assign the styles of the cells twice (inline to the style attribute and a second time via the class)?
Why are you using a class at all? A selector such as `#grid > div > div' would be easier.


Here is the simplified code I used: http://jsfiddle.net/jJa8L/2/

Some experimentation show for me that the biggest speed hog seems to be the absolute positioning. When I comment it out, then I get speed improvements of 30 to 50% (40-45 fps) in Chrome. I haven't had a chance to try it out, but I'd try using a table layout (either using a &lt;table&gt; directly in the HTML, or using display: table-* in the CSS).

EDIT: I just discovered one certain optimization: Moving the font property from the class to more global rule (body or #grid), raises the fps in Chrome to 35-38. See http://jsfiddle.net/jJa8L/3/

EDIT 2: Ok, here's a version with table layout: http://jsfiddle.net/jJa8L/4/ It's still at 40-45 fps.

EDIT 3: No idea why I didn't think of this before: Simply let the cells flow with display: inline-block giving #grid an appriopriate width. http://jsfiddle.net/jJa8L/5/ This is  slightly faster (42-47 fps).

EDIT 4:
I just had another idea: There is a considerable speed up in Firefox and IE (but not Chrome for some reason) if you hide the grid while updating the cell texts: http://jsfiddle.net/jJa8L/8/