When the Enterprise hits warp seven after Spock's engine repairs, we see those exact same visuals: the ship moving slowly on the foreground, stars (rather than streaks) in the background. So we can't tell whether the Klingons were doing warp or not. FWIW, their engines are never shown glaring particularly brightly even when they are doing high warp (see e.g. "Flashback"), even if the star background effect in other movies and shows is less ambiguous on the issue of warp vs. impulse.
It seems quite odd because the streaking stars is Star Trek's way of saying, "this ship is travelling at warp". However, this was not the case in the original series and not in this movie. Another way of showing a ship is at warp is by showing the ship scream past the camera, the three Klingon cruisers are just strolling past, with even a close up of the bridge (or what I assume to be the bridge, the little tower on the end part of the "neck"). So I think that if the Klingon cruisers were travelling at impulse, then maybe so was the Enterprise.
This movie seems to have an idea of impulse being really quite fast, such as when the Enterprise flies through the solar system and past Jupiter, personally I think this looks a bit too fast for impulse, and this makes me think that perhaps the Enterprise is moving at impulse through V'Ger.
Is there any dialogue suggesting they are at warp or impulse when they are in V'Ger?