"Felt like it took a lot more effort" is very subjective - do you have a heart rate monitor? If so, see if your heart rate was really higher on the road bike. (A power meter is even better, but heart rate is still a step up from perceived exertion.)
Another possible issue: do you have the fit dialed in on the road bike?
One thing I found when I went from years on drop bars to my first hybrid and then 10 years later back to drop bars: just changing the riding position I was used to changed my efficiency pedaling. Years of riding leaning further forward kinda tuned me to pedaling in that position and going to the more upright hybrid position "de-tuned" me until I "re-tuned". Wasn't quite so bad when I went from the hybrid to a touring bike, but did feel "de-tuned" again.
I don't often ride all the way down in the drops, but you can feel the same thing on the road bike going from hands on hoods to hands on drops - even though I'm reducing wind resistance, not use to pedaling in that position and it feels harder. The heart rate monitor shows I'm really not working harder at all though, at least from my heart's point of view!