And there we must break from the convention of modern warfare. I mean no disrespect to soldiers but they are not as inclined to fight melee as in historical battles. 40K is more akin to this begone era of muskets and swords. So a pure ranged engagement does take longer but not by much.
Few who serve do so without some recourse to the past.
The muskets and swords of which you speak are perhaps best illustrated by the Battle of Culloden Moor.
A relatively simple engagement where the Highlanders lined up, got shot at by arty for 20 minutes then charged, broke the line were counter attacked and then broke and fled. That took about 3 hours.
I think is a pretty close analogy to a small game of 40k which follows a similar pattern of advance under fire, close with the enemy then either win or lose the hand to hand.