It's a retreat march - the timing is the same as 3/4 marches like "Lochanside". The original function of those marches was for the soldiers to march back to barracks to. So, not very slow. (If they were in a real hurry to get back to barracks they'd use "The Black Bear").

The main thing to remember is that isn't a lament. It's often played by English melodeonists under the impression that it ought to be, and they just make it sound unlistenably turgid. Not helped by them usually doing it in G.

If you want a real lament marking that battle, try G.S. MacLennan's "Sunset over the Somme" - even "lament" understates it, it's a bleak scream of despair.