I've noticed that iTunes store is always slow, at first I thought it's just because iTunes is a bloatware (which it is, on both Mac and Windows.) However I started to wonder whether this is intentional -- to control the overall pace a user browses the store.
Imagine a brick & mortar store -- would the shop designer want shoppers to enter and walk as quickly as possible between shelves, or rather design the route so that shoppers are encouraged to stop, look, turn, discover... etc? IKEA is famously known for making all shoppers go through its designated route and hoping them to buy more than what they originally want.
Of course you risk the shopper leaving the site, but that's always a problem even if your site loads instantly. Obviously this is NOT a technical question. Assume the visitor is already interested in your products and want to find out more. Perhaps making them wait for a bit -- teasing them -- will make them want more?
Is there any study on this?
Additional Information (2012-3-7): the website I'm talking about is a website selling art. It is different to Amazon or grocery sites in a number of ways, e.g. each individual item has relatively high price tag, and you do wish the customers to take time to look at the art, rather than browsing through text description, reviews, related products etc.
I agree that the check-out process should definitely be as quick and smooth as possible once they've made the decision to buy. My original intention was to investigate the pace at which they move from one item to the next -- where pause is part of this overall sales presentation process.