In "Tin Man", the Enterprise-D (and her Galaxy-class sister ships, presumably) is referred to as the fastest ship in the fleet. The original Enterprise held numerous speed records, according to Captain Stiles in TSFS. It's possible that the geometries of Starfleet's heavy cruisers are better suited for high warp speeds. The modular pod of a Nebula might make it more suitable for urgent, extremely mission-specific roles.
I'd give you that she's the "fastest in the fleet".. Not the quickest mind you. Just the fastest. I have zero doubt that there are "quicker" ships in the fleet. (SFB: Federation Express). However, in terms of long range nothing could touch the Galaxy Class.
I would also think that the cost of these "super starships" would have been through the roof. Much like the Supercarriers of today.
Theres a reason we only have 10 active right now.