Does anyone know why we are still on 3.1, which was released in 2011? I thought Nexus devices got all the good stuff first... Or are only custom roms and kernels using 3.1?
Android devices rarely get new kernel versions anyway since the kernels tend to be customized to work with a specific device, and the binary drivers are built for a specific version of the kernel. This is not as bad as it sounds tho, since a lot of stuff can be backported meaning you get functionality from a newer kernel without the actual kernel version changing. Even more common with custom kernels. For example there's ROMs for our device that uses the F2FS file system which first appeared in the 3.8 kernel and gotten big changes every version after that, and it runs just fine backported to the 3.1 kernel.
there are architecture-specific commits for 3.10 in the kernel/tegra project, which points to development for the 2012 Nexus 7.
As I said, lots of the improvements from newer kernels have already been backported so there wouldn't be as big a difference in performance as you might think. The tegra commits are interesting, but sadly does not confirm anything. For example, the android police article on those same commits mentions that screenshots from the nexus 4 and 5 with the new android version still show them on kernel 3.4. The chance that the 2012 nexus 7 would get a kernel update while the nexus 5 seems awefully slim. I hope I'm wrong tho, since I think it would make things simpler for the custom kernel developers to base stuff on a newer kernel but I wouldn't get my hopes up...