The crude way their ships appear could be a culturally driven look, deliberately cramped, misty and with low light levels. When Kirk ordered his ship out of Romulan space at warp 9, the D-7 chasing him was able to match his speed.
One of the things implied in TNG, is that one of the aces in the Federation hand that helped bring about the permanent Klingon/Federation alliance, was the superior Federation engine technology. In "Yesterday's Enterprise", for example, the alternate Picard notes to his crew that they could outrun the Klingons, but they needed to cover the Ent-C while it reentered the wormhole. This shows up in a few non-canonical novels, as well.