When you're happily fishing and your boat suddenly changes direction any speed feels like a fast speed. The speed is not estimated in the article. Fortunately the Irish Sea is not that deep only a few hundred feet in most places. It looks like the sub was operating in the deeper channel on the west side which is still only about 400ft in that area. Hence he downward vector maybe a little less. If this was a Russian sub interested in NATO exercises it would have been running at tactical speed, (say) about 25 knots to avoid shallow water detection and it passive sonar array should be optimized to that speed. The array on the tower is usually a 360 array. If they detected a constant bearing/range surface sound, it more likely than not they'd bother to do anything about it. Unlikely to pop up and wave to satellite, I think. Lucky lads! I'm glad.