最近在看 ios 文档Concurrency and Application Design，关于 Dispatch Queues 中的介绍有几句，理解起来很费力，求大神帮忙解释。

上文是这句： Dispatch queues have other benefits:

看不懂这三句：

1. They provide the speed of tuned assembly.
2. They do not trap to the kernel under load.
3. They scale gracefully under contention.

谢谢