It's not that CPU-intensive. On a typical hardware I doubt it's even measurable. And the kernel is preemptible, so it wouldn't matter much anyway. And, if you don't like it, you can disable it. Point is, functionally, there are very few things ALSA does better than OSS, despite lots of added complexity.
I have no idea how the situation looks like with V4L, or any video, for that regard.