Answer:
Processing speed is measured in Gigahertz (Or megahertz).
It is very important because processing speed is a measurement of how fast your processor (CPU) can process data, and it gives a good idea of how fast the computer can run. There are other factors too, such as RAM and the video card for graphics performance.
It's also important to consider the kind of processor inside the computer.
There are some built for especially for laptops, and processors can be classified by how many cores they use. A dual-core processor has enhanced performance over a single one, for example.