Over the past couple of months, I've been pushing the boundaries of the Data View Web Part (click here for a good "how to" if you're not familiar with it). It allows me to pull in data from multiple sources and present it on a page in tables that literally draw themselves.
The challenge is that data is data and the more you throw on the page, the slower the page gets. I've found that what only takes a few seconds to display when viewing the page on the web can take several minutes in Sharepoint Designer to load up to edit. That's a long time when you're cycling through a process of making small changes, saving, and viewing over and over again.
Luckily, I stumbled on a solution - when you click on the small arrow on the right side of the web part, it brings up a menu of common data view tasks. Simply check the box marked “Show with sample data” and the web part will use sample data while you’re in design view instead of connecting to and loading your actual data every time the page loads!
The only down side that I’ve discovered is that sample data won’t load for input boxes on the Data Form Web Part. Still, the load time on my page has improved considerably!