I suggest respectfully you read the manual on Preview.
If it is slower than you think the available horsepower should dictate, this is almost surely because the horsepower is not available.
If you had complied with the "Read this before you post" thread, we would have been in a better position to know more about the graphics you have, wouldn't we? Press this button to see it now:
Yes, if you have an on-board graphics system, it'll pinch a massive dose of RAM that would otherwise be available. If you have a proper graphics card, no, it should make no difference.
A video profile (or, better, a dual boot) is nothing to do with who uses your computer. It has everything to do with fine-tuning your computer so that it gives best performance for video without interference from other stuff on your computer.