The top function takes 50% of the runtime,
and that means that even if you made it go to 0 time,
the program would only go twice as fast.
So you'd get a 2x speedup.
The top 3 functions between them take 50%, 20%, and 10%,
or 80% of the total time.
So if you were to make those all drop to 0,
the program would be taking 20% of the time that it used to take
and you'd have gotten a 5x speed up.
排第一的函数占用了50%的运行时间，
这意味着即使你让它的运行时间为0，
程序只会运行快一倍。
这样你就可以得到2倍加速。
它们中排前三的函数占用总时间的50%、 20%和 10%的时间，
或80%。
所以如果你能让这些都降到0，
这个程序将只花它原来时间的20%，
你会得到5倍的加速。