I think speed depends more on cargo than other stuff, I used to have a speedy bike and a sluggish workhorse and the more load I transferred to the speedy bike the more of a sluggish workhorse it became. Now I have a roadbike w/ no rack so I cannot have cargo creep (unless I go from a shoulder bag to a backpack I guess). The other 2 sluggish workhorses, well, one is pretty redundant but it does have some features making me keep it around for a while longer.
To attempt to answer your question, on one sluggish bike I can maintain 14-16 mph, on my new (to me) road bike I can push 17 mph sometimes, but not too often; w/ traffic and lights over 7 miles that's like a 1-2 minute savings. On that same road bike I can exceed 18mph average over much longer fun rides w/ no cargo and few lights.
Commuting is just different, so a 'properly set up road bike' will give you a max speed on long ride conditions that you will pretty much not be able to approach while commuting. And if you load your road bike the same as your mtn bike your speed will probably be identical.