People who say they can keep with other riders on normal size wheels and tyre are clearly fitter than they think .
I’m 100kg, 38″ waist trousers and think of pork pies as a nice snack but not really enough for a meal.
You would have to try pretty hard to consider yourself less fit than me!
It’s slower, but not unreasonably slow. My KOM on my commute is actually on my fatty, previous best was on my XC 29er. I’m not saying the fatty is faster, it’s probably more to do with getting lucky with junctions and chasing someone down on the final mile, but it’s not ‘slow’. Best analogy would be riding a rigid bike with soft compound DH tyres, in fact I don’t even think it’s that slow.