OK, grab a pinch of salt for this one, because it's taken straight from the box marked "Rumour".
Rayman 3D, Sonic Generations and Ridge Racer 3D are just a few of the titles which have allegedly been enhanced by the 5.1.0-11U update, and it's also being reported that the Mii Plaza is less jerky now — something we've personally checked and can't quite decide if there's any discernible difference.
Given that many posters in the original forum thread are questioning the evidence, this could well be the product of some over-active imaginations, but Nintendo did speak of underlying improvements being part of the last update — perhaps some kind of technological wizardry has been performed which does in fact result in slightly enhanced software?
Have you noticed any difference since the update was applied? Or does everything feel exactly the same to you? Let us know by posting a comment.