This is all anecdotal, but a couple of decades ago I was riding road bikes with standard 622 rims shod with 23 mm tires. In the 2000s I shifted to riding road bikes with 584 rims and 42mm tires because they were frankly just more fun for me (lower pressure = greater comfort and better suspension meant better handling on our rough Northwest roads). The outer diameter was about the same, so I didn’t have to do something weird with the geometry of the bikes.
When Jan came out with 55mm super-supple tires on 559 rims (again that would keep the outer diameter the same) I thought what the heck and got Seven Cycles to build me a custom around the new tires. Now that road bikes run disk brakes, you can run multiple rim size on the same frame so I figured I could always go back to 584/42mm if I didn’t like the crazy wide tires. (I was worried enough that I actually got somebody to build me a new set of 584 rims for the new frame.)
I’ve had that bike for about 3 years now and have ridden several thousand miles on it. Only a few tens of those miles have been on the fancy 584 rims with the (equally nice) 42mm tires – the 55mm tires are even more fun than the 42's were for me. If you ride on mixed surfaces, the extra suspension you get from running low pressures (I run around 26 PSI front/32 back) makes a big difference (or at least it does for me) - both in handling and comfort.
Downsides are increased weight (the 55mm tires weigh about 420 g a piece which is impressively light given the amount of material, but still a lot heavier than those old 622/23 mm tires back in the day) and the increased maintenance of running them tubeless (though of course you can run them with tubes-though given that I haven’t had a flat in 3 years and I’m riding on Seattle streets, I’ll put up with having to monitor the sealant levels every month or so).
I am definitely slower - 4.5mph or so - on my 42mm gravel bike on road than I am on my 32mm road bike.
But my road bike has slick tyres and the gravel bike’s tyres are more knobbly, and I run the gravel bike’s tyres at much lower pressure. (And there is the difference in geometry of each bike, too.)
I’m not even sure you can buy 42mm slick tyres, so a like-for-like comparison is probably very hard.
When Jan came out with 55mm super-supple tires on 559 rims (again that would keep the outer diameter the same) I thought what the heck and got Seven Cycles to build me a custom around the new tires. Now that road bikes run disk brakes, you can run multiple rim size on the same frame so I figured I could always go back to 584/42mm if I didn’t like the crazy wide tires. (I was worried enough that I actually got somebody to build me a new set of 584 rims for the new frame.)
I’ve had that bike for about 3 years now and have ridden several thousand miles on it. Only a few tens of those miles have been on the fancy 584 rims with the (equally nice) 42mm tires – the 55mm tires are even more fun than the 42's were for me. If you ride on mixed surfaces, the extra suspension you get from running low pressures (I run around 26 PSI front/32 back) makes a big difference (or at least it does for me) - both in handling and comfort.
Downsides are increased weight (the 55mm tires weigh about 420 g a piece which is impressively light given the amount of material, but still a lot heavier than those old 622/23 mm tires back in the day) and the increased maintenance of running them tubeless (though of course you can run them with tubes-though given that I haven’t had a flat in 3 years and I’m riding on Seattle streets, I’ll put up with having to monitor the sealant levels every month or so).