I have only been cycling regularly for a couple of years. I spent the first 18 months on my Kona mtb, I used it for both off road then more on road by fitting 47mm road biased tread tyres. My average speed gradually increased to 14.5mph over 40 mile circuits.
Then last winter I decided I would try and ride 100 mile sportive events this spring/summer. I got a Boardman cyclocross bike mainly due to me liking hydraulic disc brakes. I really like the bike and fitted 32 mm schwalbe marathon plus tyres. They were comfy and rolled quite quickly compared to what I had been used to previously. I am thick set built chap weighing mid 15st but carry a lot of muscle mass (early youth body building:) )
My average speed went up immediately obviously a lighter road bike and tyres. My first 100 mile sportive I averaged 16.5mph. I could then average high 17mph over 30 miles. A few seasoned riders on the sportive mentioned I would go quicker and it would be a little less effort if I moved down to 25mm race tyres
Yesterday I had my first outing on Schwalbe Durano 25mm . Over a 28 mile route I averaged 18.40mph. A noticeable jump in average, my previous best on 32mm tyres was just 18mph (optimal conditions). I don't think I could get any better results by moving down to 23mm, I don't think my weight/body could tolerate the harsh ride.
The ride is slightly harsher with 25mm but I find I can hold a larger gear for longer. Has anybody been through this learning curve?