15mph versus 13mph is about 15%, which is an absolutely gigantic performance difference to exist between two bikes. Like, even when I'm climbing steep hills, I'm only about 6-7% faster on my 20lb road bike than on my 33lb gravel bike, at similar effort.
going from my 20lb gravel bike to my 17 lb gravel bike I'm consistently 10% faster climbing. But I'm thinking there is more going on here than just weight. That lighter bike is crazy efficient compared to the typical bike.