Speaking on fast turn-around and conversion rates, we would talk on how easy would user get from the front page into checkout confirmation. Normally they would meet lots of obstacles to overcome and may easily become frustrated, especially if time is value. Although there are number of usability resources across the internet, dedicated to user-friendly UI design, developers and designers are still failing to deliver nice experience to user.
Why is that? Do they do this on purpose? Well, in most cases - yes. Every site owner wants their solution to differentiate with either cheap stuff, or more features and sophistication of tools or experience. The type of latter customers envision user as savvy computer hacker that wants all available functionality at once. Getting down to online design this might mean whole bunch of special and/or non-standard flows for product design like trying to upload own design image that tool has to understand in some way, then add numerous texts in various effects including distressed text with white outline put into bridge effect. Unless the above is needed for very special bulk order of some university football team, it might not be needed all at once for an average user who wants a simple "My Name Here" t-shirt.
The most simple equation that is worth considering is time = number of clicks. Less clicks, less time, more conversion. Modern users hate to click or type a lot - they want straight, simple and fun experience. They want the tool or site do the orders for themselves, read their mind without having them difficulty explaining support what they want. That's it - the recipe itself is simple enough.
For site owner this would always look as complex dilemma. Why should I sacrifice all the features I wanted to have on my site? Wouldn't I look simple comparing to my competitors? To tell the truth, such dilemma does not have a straight solution. What you'd always have to keep in mind is that whatever functionality you put into your tshirt design site or online product designer solution, it should be easy on user. Simplicity if the key if you want them to return again and make an order.