If my bike can fit a wider tire I am going to put the widest tire I can on it. Speed is only fast if you are comfortable. Skinny tires just don't have much comfort they just feel fast because you feel more of the road. A wider tire can be faster and if you are more comfortable on the bike you can go faster and put more effort in. Plus you get the added benefits of lower tire pressure and less chance of flats which is always a good thing. My current hybrid is running a 38mm Continental Travel Contact front and rear and they are fantastic tires, still the same ones I had when I built up the bike. I think the frame maxes at about 40 but with fenders I lose a little space but don't want to ditch the fenders so 38s it is.

My skinniest tire is a 25c and I tried 28s on the bike and they fit a little better at the front but the rear they would rub and the paint is in excellent condition and is rare enough I said ok go narrow and live with it but it is the bike I enjoy riding the least but enjoy looking at the most so go figure.