也就是在当前所有的功能模块中做一下“切片”，切出来的片可以典型地代表系统的性能，如果这个切片能通过测试，那就可以认为系统性能可以满足要求。
那么怎么样决定哪个功能属于切片呢？"Often the vertical slice will implement some of the functionality that we suspect will have the worst performance in the application and some of the functionality that will be most heavily used in production"