Two reasons.
First reason is translation. In terms of emulation, not all systems are built on the same architecture. In basic terms you can think of this as a default baked list of operations in a chip. That means they have to be translated over to run on your chip, and this translation is done in real time. This is a performance hog.
Second reason is individual core speed. Ive noticed cpus went more in the direction of higher core counts over clock speed. Base clocks as low as 2ghz. Especially when you are doing those translations mentioned above, clock speed is important. A 4.5ghz quad core would slap a 3.2ghz octacore out of the water for a task like this. This is especially a problem with devices like laptops where thermals are infamously bad and requires the cpus installed often to run at the base clock, which is often in the 2ghz range, unless you want your laptop to turn in to an aircraft and lift off.
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