Some are interpreting your question as "if I switch from a 'road bike' to a 'touring bike' on my weekend road rides, how much will my speed change?" Instead, I'm interpreting it as "how much will my speed change between weekend road rides and long-distance touring?"

I use the same bike (a 'touring bike') for commuting, weekend road rides, and touring. So the difference is in the additional weight and wind resistance added by strapping 5 bags to the bike for touring, and perhaps in the increased/repeated distances. When in unloaded, weekend-ride mode, at the end of a 50 mile ride, my cyclometer shows an average of 17-18mph. While on tour, the average of my daily speed averages at the end of each tour have been remarkably consistent around 13.5-14mph. The "average speed" shown on my cyclometer at the end of the day ranges from 10mph to 16mph, depending on wind, hills, and my energy level.

So it's a 20-25% speed dropoff for me. If you plan to carry less than me on your tours, your dropoff may be less.

For someone who hasn't toured before, but has experience with other types of bike riding, this seems like an entirely reasonable question to me, because it's a way to gauge how much ground one might be able to cover on a tour in a limited time period.

Neil