Yes, the Linux kernel is, but not all of the userland, which is what makes Elementary OS, "Elementary OS." ElementaryOS outside of the kernel is primarily Vala[1].
One of the comments by the ElementaryOS developers the other month on HN about their usage of Vala and its performance:
> aroman 51 days ago | link
Vala is, all in all, fantastic. The way I describe it to people is "it feels like C# but runs at C speed", and that's pretty much accurate. The only issues we have are binding bugs and Vala bugs, both of which are harder and harder to find these days. And when you do find one, the development community is very active and you can find the help you need.
For concurrency, I have personally written async vala code (which has some excellent support, including closures), but there is full thread support via GObject. The thread safety stuff is the same as the underlying library, and lots of work goes into GLib/GTK+ to make them thread safe where appropriate.
