I don't necessarily think it has to be that much faster, because the Sonic Rush games on the DS are the fastest and I think they're too fast with not enough platforming. Sonic 2 is my 6th favorite game all time, I want something paced like that, fast, but still a platformer.
I think we're mostly in agreement that Sonic Adventure 1 is the best 3D game in the series, well it was the slowest one, too, at least of the ones I played. It essentially copied the style of Super Mario 64, a hub world off of which you accessed different action stages which each contained multiple challenges. There was some speed, and some exploration. Too much speed gets you Sonic Rush, too much exploration gets you Knuckles and Rouge in Sonic Adventure 2.