Seat tube angle on that bike is a little slacker than a road bike and even many hybrids or fitness bikes. So that puts you at a power disadvantage where for the times you might have to go hard, you are doing more work than others with their body weight more above their legs.

If you are in good shape then the Verve still might have been a very relaxing 12.6 mph average. But if you've had a rough season like I have and haven't ridden as much as you should, then you'll struggle on any bike.

However my choice would be

No one thing; it was a combination of those things
AND
Other

Even more other things than what I wrote here. Though I'd seriously recommend getting rid of the knobbies if most of your riding is on pavement. Fit your bike out for the majority of the riding you do with it. Not the things you might do just a few times a year.