It really depends on what you are doing. For my moodle setup the CPU basically idles all day except when doing the backup. My installation isn't as big as most people's though. To quote a source: "General rule of thumb for hardware upgrades: For PHP scripts, the main bottleneck is the CPU. For static HTML/images, the bottleneck is RAM and the network."
Hardware and performance
Hardware Recommendations for Apache / PHP server
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