my super wild arse guess:
In my limited experience its always been the disk cache thrashing... at the end of the day, it will prolly not fit in real memory... and will have to page out to the disk.
- run the test, see if its hits the HDD... ie. check to see if the HDD light is blinking when the test is running)
more than likely it wont hit the HDD... it just inspecting info on your hardware rather than running an actual load on the system to see if it passes.
- then run the game... odds are if you dont have a high end graphics card, its gonna off load alot of the stuff intended for the GPU and its ram to your CPU and your mobo's ram + disk paging...
If you are getting disk hits during game play... swap out your HDD for a SSD... or at the very least defrag the drive that has the page mem allocated on it.