I think the increase in desktop graphics performance has to do with the updates in Mesa 3D Graphics Library. Ubuntu 13.10 has 9.1, while 14.04 has 10.1.
Though I would have thought that Intel HD 4000 would provide decent performance even with the old Mesa. I'm on GMA 4500MHD (GM45, ThinkPad X200, circa 2008) and that feels laggy sometimes (with all special effect options I could find tuned to a minimum), especially when I compare it to my brother's desktop with gaming-grade video card. Most times I put up with it, but one time I decided to try the mesa 10.2-devel, and suddenly - what a relief, my old laptop finally feels like it has no problems displaying windows, again! There are some glitches when displaying some window decorations, but I'm ok with that, as long as the smoothness of work is back (I guess, that's the artifact of -devel version).