To do its job, Photoshop needs to process bit after bit of image data as fast as it possibly can, so having a powerful CPU obviously helps.
What exactly does "most powerful" mean? You can break down this concept into a few basic factors:
Gigahertz. The most obvious measure of CPU speed is gigahertz (GHz), the number of cycles per second that a CPU can handle. Computer companies advertise gigahertz because it's one number that can be increased in ads over time to make it seem like it's time to buy a new computer all over again. It's easy to use gigahertz to compare CPUs of the same type, but gigahertz is less useful for comparing CPUs of different designs. Gigahertz is not the most reliable way to compare whole computers, especially of different brands, because there are other variables involved such as the speed of the disk, RAM, and video. You might think of gigahertz like revolutions per minute (RPM) in carsmore RPM can mean higher speed, but it depends on the car, the engine type, the gear you're in, and so on.
Number of CPUs. Some high-end desktop computers contain two or even four CPUs (Figure 1.1). Two CPUs are better than one, but only if your programs are written to use multiple processors. Photoshop can use multiple processors, and so can Adobe Camera Raw. Some older Photoshop plug-ins may not be multiprocessor-aware.
Figure 1.1. Processor configurations found in today's computers. From left to right: Single processor, dual processors, single dual-core processor, and two dual-core processors.
Number of cores. Up until recently, the only way to have multiple processors in a typical computer was to add another CPU. Now, an increasing number of consumer-level CPUs have two processors in one CPU, and this is a very good thing. To distinguish between the number of CPUs and the number of actual processors, the processor inside a CPU is called its core. Dual-core CPUs have two processors in one CPU. Dual-core CPUs save space and are just as fast as having two single-core CPUs. If you use Photoshop on a laptop, rejoice: Thanks to the multi-core design, you can finally buy a reasonably priced laptop with more than one processor inside, enhancing performance significantly over traditional single-processor laptops.
If you can afford it, you can go all-out and buy yourself a desktop computer with two dual-core CPUs, giving you four processor cores to throw at your work. Of course, you'll enjoy their full benefit only during peak loads. But if you do spend a lot of time converting Raw files or running filters on large images, having all that CPU power can be worth it.
When Photoshop doesn't need all of the processors you have, Mac OS X and Windows XP are both able to apply unused processors to other programs you're running. If you have only one processor, it has to handle everything running on your computer, from processing a folder full of deadline-driven images to flashing that cute alert icon in your chat program. With multiple processors, the system can dedicate a processor to one task and move other tasks to another available processor, allowing the first task to finish faster by letting it work without distractions.
Will a Faster CPU Help?
Before you go shopping for a new CPU (or two) to speed up Photoshop, check your machine to make sure it's already working the current CPU to the limit.
On Mac OS X, open Activity Monitor and click the CPU tab at the bottom of the Activity Monitor window (Figure 1.2). If the CPU Usage graph indicates very high usage (or low Idle percentage) as you work in Photoshop, that's one indication that the CPU may be at its limit. In the main Activity Monitor list, sort the list of processes by CPU usage by clicking the CPU tab to see which applications use the most CPU power.
Figure 1.2. The Process list and CPU Usage tab in Mac OS X Activity Monitor monitor system performance on Mac OS X.
On Windows XP, right-click the taskbar and choose Task Manager. Click the Performance tab (Figure 1.3) to see CPU performance statistics.
Figure 1.3. The Performance tab in Windows Task Manager monitors performance on Windows XP.