Even as we can expect Ubuntu 13.04 to use kernel 3.8 next April. Even as users can look forward to many improvements in kernel 3.8, the Linux kernel seems to have a certain problem when it comes to desktop responsiveness. With each successive release of your favourite Linux distro things just seem to get worse.
When the desktop runs fine, it's smooth as silk. When it's not... well, it stutters a lot. This is apparently due to the CPU scheduler. And now we're seeing a new CPU scheduler appearing. Read more.
It's just out so there's really no indication of how this new scheduler will perform yet, nor do we know if it'll get included in the kernel even. It's in a very early stage of development.