Depending on where you live, finding a shop that stocks touring bikes for you to check out can be frustrating. If such a shop exists within a reasonable distance, a visit could be educational.
Yes, a typical touring bike will feel noticeably slower...even sluggish at times...compared to today's common road bikes. Some of this has to do with weight, while some of it has to do with geometry. The frame is typically heavier due to its more robust construction. Add to that the extra weight from heavier wheels, much heavier tires, racks, a proper saddle and the other overbuilt bits and pieces, and you have a bike that can weigh up to twice that of a competitive race bike.
The geometry will have you in a less aggressive riding position. This can also contribute to slower speed. But it is this same geometry that will make you comfortable as you roll for miles and miles, day after day. Add the right gear selection for your terrain/load, and you'll soon adjust to the slower pace.
Also, keep this in mind; the extra weight of the bike will seem small in comparison to the load you'll add to it for a long tour.
As for bikes themselves, there's much more available than the 520, though I'm not knocking it. There are various complete tourers on the market, in addition to many more framesets for custom builds. Most are steel in various types of tubing. Rocky Mountain, CoMotion, Bruce Gordon, Rivendell and Surly are just some of the makers of frames popular for touring.
Whatever you get, make sure it fits you nicely. It should be comfortable. Get the right saddle for you, be it Brooks or something else. Make sure you can get the bars high enough, which will be difficult if you get too small of a frame. Touring frames typically have low bottom brackets and slack seat tube angles, so you might find yourself on a *larger* frame than you would for a typical road bike.
Also, getting a wheelset handbuilt by a good builder can bring a lot of peace to mind. As stated by someone else, 36-spoke is great, though heavier riders or those carrying heavy loads over bad roads may want more spokes. Some of the wheelsets I've seen on complete touring bikes are built (machine?) with components I wouldn't feel comfortable even commuting with. And get some good tires...like one of Schwalbe's Marathon models.
So, if you can spend the extra money up front, selecting a frame and quality components for a custom build might be ideal. When you have something that fits you perfectly and is assembled with your choice of components, you might actually enjoy the more relaxed pace of a touring bike.
Good luck and do your research...