It is only in TNG "The Enemy" that we first truly learn that the scale of things might be different. There, it takes a modern, definitely high-warp-capable Romulan warbird at least an hour to cross from the inner surface of the RNZ to the outer...
Which makes me think the Romulans did have some sort of FTL capabilitys on there BOP.
I liked what someone else on here said. Maybe it had a singlerity device like in the later series but as it was a new tecnology the Enterprise just couldnt detect it.