The reasons can be anything, from hardware to software to whatever!
* bad bios settings
* graphics card
* graphics memory
* type of memory
* amount of memory
* HDD access speed
* HDD seek speed
* HDD free space
* drivers
* cpu
* windows version
* programs installed/running in the background
* monitoring programs
* corrupt or too small pagefile
* registry settings
* windows services
* etc...
* etc...
* and the list can go on and on and on and on and on .... and on
Computer speed is extremely relative and it is by no means that a faster cpu means a faster computer.
I can own a big expensive fast car, yet when I don't perform regular maintenance and don't know how to set things correctly, a fine tuned DAF could still win on the race track...
/ 
