I have bike computers on my Trek 1200 and Trek 520.
When on the 520 with just one pannier, I usually average about 12.5-13mph.
On the 520, with just a seat pack, I can go about 16-16.5mph.
Differences: The 520 weighs more, has fatter slower tires (700x35/70psi vs 700x25/100psi), has flat handlebars and an upright posture, longer chainstays, etc.
And the pannier creates more drag. And I'm usually in a different mindset riding the 520 vs the 1200. I recently did a 500+ mile tour on the 520 and averaged over 11mph on a rails-to-trails surface hauling tent/bag/food/supplies/etc. The 520 doesn't go fast for me, but it doesn't go much slower when loaded down, which makes it a good loaded tourer.