Bug 123989 tracked improvements in laying out grid items in large grids. Next step is to reduce also the painting time.
The idea is to try to use our knowledge about the grid structure in order to perform the smallest amount of paint operations as possible. This was already being done in blink, the tracking issue is this one http://code.google.com/p/chromium/issues/detail?id=248151.
My idea is to merge Blink's r155262 and r157993. Regarding the issue with overflowing grid items I proposed a better fix in blink herehttps://codereview.chromium.org/98563003/. I'd follow that approach in WebKit too.
Setting a dependency, we need to correctly deal with spanning grid items before applying any optimization.