It is generally accepted that the spd sl pedal benefits from less energy wastage from movement, twisting and vertical. The shoes are generally lighter and stiffer. With regard to bike cost, I always tell people to buy the most expensive well specced bike they can afford. If you don’t and you actually enjoy cycling you will be changing very quickly. Carbon frames are stiffer, lighter, and faster. Ultegra gearing has less friction, cleaner changes, and more precise. Expensive wheels are more aerodynamic, weigh less, and are stiffer and faster. Better brakes stop quicker. For those who think there is little difference between a £600 bike and a £5000 bike have obviously never ridden one. Even the difference in a £2k bike compared to a £600 bike is night and day.
From my experience I find that on longer rides, my toes and feet start cramping. The positions of the cleats and width of the pedal surface is to blame. Once the cramp sets in it will be slower. Was on the Shimano M520 then switch to the wider M530 over 4 years. Same problem. Don’t have the issue on my spd sl.
Carbon is stiffer and gives a smoother ride, it's not just about weight.
Assuming same shoe sole construction and weight how is it faster exactly?
Sure, but as an alternative view, I bought my winter road bike used for £600 and it weight just over 7kg, so I would equally question why anyone would spent so much on a new bike. Of course, the answer is easy - it's because we all have different priorities in life and value things differently. Some would want a nice, shiny new bike that never needs to be touched, while others sill spend (quite literally) hours in the workshop tinkering and repairing them. I'm one of the "club riders doing 200 miles a week at 19mph" mentioned above and I really value my cycle kit. I've got a good idea on where to spend the money and where not to and I definitely appreciate the difference between a £500 bike and a £5000 one! Not everyone will feel this way, but that's ok - I don't like golf, football or any one of a hundred other things that people enjoy doing. I like cycling and am happy to spend money on it!