Comparing my TT bike (700c 23mm tires), a few 451 mini velos on 28mm and my newly setup gravel bike (650b 42mm Pari motos), the 650b with 42mm feels very different.
I ride mostly with/vs roadies for the local rides here and the gravel setup is really for overseas bike touring, but I like to see if I can get away with.
Pros
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1. I can largely ignore most road irregularities.
2. If there are climbs or descents that also have bad road surfaces, the 42mm tires are better
Cons
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1. Climbing vs lighter roadbikes (or my own TT bike) on good roads is certainly slower and requires more effort.
2. Speed drops by about 3-4kph
3. Certainly more effort needed to get to speed (and keep it)
4. Slower acceleration
Other notes
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1. Though I have been able to keep up or even get ahead of other riders on road bikes, I know that it takes more effort to do so and becomes hard over a longer ride.
2. Speed is not a real problem riding alone with a loaded bike (so maybe comfort takes over as a priority? )
3. 42mm does not seem to be the right width for road riding unless one rides in beaten up roads most of the time.
4. I'll try 28mm tires soon to see how the weight/speed/comfort equation works out.