Different permutations of wheel and tyre can make a massive difference, I currently love riding my giant defy with old 2010 fulcrum racing 7 wheels and continental grand prix (24c) tyres, but change those wheels to something heavier (pro-lite como) with 25c tyres and it changes the feel of the bike, much more comfortable, higher speeds are easily sustained when you get there, but it's harder work to get up to speed.. Go the other way with lighter wheels and tyres (campagnolo zonda with GP4000s) and yes it's really fast, easy to get up to speed, but much less comfortable, doesn't handle the tiniest bump in the road surface well.