I have a cyclocross Trex XO-1. The tires on it are knobby Bontrager Jones Cyclocross tires (https://www.bontrager.com/model/00433/) They are 34c. Would upgrading to some road hardcase tires be better for commuting in terms of speed? My commute is strictly pavement though some parts are bumpy and rough due to cracks, potholes, etc. I am thinking of getting Bontrager Race All Weather Hardcase tires (https://www.bontrager.com/model/09184) at 25-28c. Basically how much faster/easier will my hilly commute be?

I think you'll find if you go to a smoother, lighter, thinner tire your avg speed will increase, maybe 2-3mpg ....depending on variables. I do like Bonrgr Hardcase tires, big thumbs up, but I don't know about the AllWeathers. As an all around commute, training, touring tire I lean toward Gatorskins. Maybe a tad heavier, but the 28s are impervious to bad roads.

I ride with 28's. They handle road problems well and help with an increase in speed.

unless you buy the world's crappiest slicks made with the crappiest rubber and keep them under-inflated, the answer is almost unilaterally "yes" if your commute is 100% paved surfaces.

my bias is for the thinnest, highest-PSI tire you can reasonably put on the bike. i commute 30 miles everyday on my road bike with 23s @ 135 PSI and i love it! yes, it's a harsh ride, i can feel every single grain of sand i roll over, but i feel like i'm riding on a steel rail, and that's a marvelous feeling.

Yes, switching to slicks will make a noticeable difference to your speed. I am personally partial to Pasela TourGuards. I use 32's on my Tricross Sport, and they soak bumps fairly well and roll faster than the knobby cross tires I had previously.

It seems like I feel bumps on my cross tires as it is. Then again, I'm sure handling will improve with thinner tires too as I will be able to make small sharp turns easier. Ok, will 25s work? And is it hard to swap out the tires if I need to ride dirt one day?

Slicks also won't wear out as fast. The only downside is that once you take this leap, you'll inevitably end up buying a second wheelset for your off-road tires, assuming you ever do that. And I'm not even really sure that's a downside.

Maybe I'll buy another wheelset when I pay off the bike. What can road tires handle? Like potholes? large cracks? My biggest worry is slipping on those little rocks on the road. I was told that road tires need more trueing also.

Skinnier road tires will absorb less of the road shock, so the ride will be a little harsher on both you and the bike. It's nothing to worry about though, both you and the gear will be able to take it just fine. Even skinny road tires can handle most of your road debris, I've even had reasonable luck off-roading with my 28mm slick tires.

Check out the Schwalbe brand of tires. .

Oh hey one more question. Will a Brooks 17 saddle help with the road bumps from having thinner tires? That's my plan so far, get the seat then get the tires. Then get leather handlebar tape )