I have some old Star Trek the Next Generation fact cards about ships. The one for the Ambassador states, "it was discovered that due to its increased mass over previous ship designs, a compact spacetime driver coil would be needed for the impulse engines in addition to the one used for the warp engines."
I don't know what that means since it's all fabricated tech talk, but it would seem like the Ambassador should be a bit slow since it was such a large ship for its time, much larger than the Excelsior. On the other hand, the Galaxy was known for being very maneuverable for a ship of its size.
Of course, in all the DS9 large space battles, we don't notice much of a difference in speed. All ships seem to move at the relative speed to each other, one of the most notable in my memory is that of the Yamaguchi/Belleraphon/Saratoga task group at the Battle of Wolf 359.
