Actually it's a combination of factors, since it's the TOTAL throughput of all the components combined that make the perception of speed noticeable. Heck, even having a monitor with super fast refresh will make a substantial difference.
Multiprocessor and multi-threading processors are more beneficial when you are doing multiple concurrent tasks on your computer. Which honestly, with the way most operating systems work these days by doing house cleaning in the background, can be a substantial drain on processing power. Then there is the old adage that the more RAM the better. Which goes hand in hand with the above mentioned situation with multiple processor threads running. Each task the processor is doing is going to require RAM for each program. The important issue here is to make certain you have enough RAM for each process to use ONLY the RAM and not have to use the disk drive as virtual RAM. Hard drives are much slower then system RAM, so any task requiring disk accesses in order to operate will run much slower than if they could remain solely in RAM.
Then you have the hard disk itself. Faster is MUCH better, no doubt about it. The new SSDs (solid state disks) are wonderful for speeding up loading times for startup with the operating system and also for loading applications.
Anyway, that's kind of the short version, but you can likely see what I mean about the entire throughput of the system needing to be considered.... Even issues like the speed of your graphics card or the network interface card for internet related activities all play a role in how you perceive the SPEED of your computer.
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