When you see your databases CPU at 100%, what is it actually doing? We like to act as though CPUs run code linearly for simplicity's sake, but in truth CPUs are incredibly complex beasts; they can potentially do many instructions at once, or sometimes none at all (despite our console showing 100%). Because CPUs can do so many things, one CPU at 100% may be doing much less than another at 100%. A CPU is considered "idle" only if no task (Linux task) needs CPU time; this does not mean the CPU itself is being fully saturated. So why would a CPU not be fully utilized, even if it appears to be at 100%?
Memory accesses are orders of magnitude slower than internal CPU operations. To give a good picture of how much so, a modern CPU might be able to process somewhere in the realm of 4-8 instructions per nanosecond. A memory access to RAM can take somewhere in the realm of 50-100ns. To ease your mathing, that means a modern CPU can do around 400-800 instructions for every one memory access from RAM.
