I have two bikes, A and B. A has drum brakes (back and front), while B has rim brakes. Very soon after buying A I noticed that it was more of an effort to get anywhere with this bike as compared to B. Both bikes have broadly the same kind of tire, and are inflated properly.
So I took bike A. I disconnected the rear brake cable, just in case it was partially engaging. I lifted the rear wheel from the ground, made it spin, and let it slow down until it stopped. I did this a few times with both my bikes. Without making any measurements, it was quite obvious that the rear wheel of A slowed down much faster than B. There's obviously some resistance in it.
So I took it to a shop and explained the issue. They put it on a stand, spun the wheels, and then explained to me that this is just normal with drum brakes - they just make the bike harder to ride, and there's nothing I can do about it.
And now here I am trying to learn more about this issue, yet none of my web searches produces anything related to the claim that drum brakes make the bike slower. So, is this true? If it is, can nothing be done about it? And if it isn't, what's the problem with my bike :) ?
I took a closer look at the components of my bike. Should totally have done this before.
On the back wheel: Shimano Nexus SG-8R36 internal gear hub
On the front wheel: Shimano DH-3R35 hub dynamo
On both wheels: Shimano Inter M roller brakes (not sure about the exact model)
So they're specifically roller brakes, not "classical" drum brakes. I found in https://www.sheldonbrown.com/rollerbrakes.html that these brakes do add a bit of drag. They probably add more drag if they haven't been cleaned or re-greased in a long time, which is the case with this bike.