I'm evaluating 3 shopping cart options of which this is one.   I'm concerned about the home page download time for my customers.  Loading in less than 3 seconds a requirement.  What is it that slows things down?  The theme? The service provider? Plug ins?  Is the Market Theme considered fast loading?  

The project is a 20+ page ecommerce and information site for products that are sold "plain" or customized with engraving or imprinting (quantity orders).  I was thinking of putting a flash sound and a video file on the home page.  

At the moment, I'm leaning towards Market Theme, I just need a little more info to get me over the fence.  The other options I'm looking at are WP eCommerce with Vanilla Cart (plus: customer history, on page shopping cart, upgrade from PayPal) and Magento with Modern (plus: reporting, expansive, very beefy).  I like simple clean themes.

My primary concern between these three is download times… any advice?  Am I off base?  Comments, and straight forward insight are greatly appreciated!  THANKS in advance!