How long would a standard 1500p game of 40k take, if you were one of the models?
Skip the turn-based move and shoot, skip all the thinking, skip the dice, skip the checking up rules.
Just run, shoot and fight.
Two small forces (rarely more than 100 models each), starting 50-60m apart, and: GO!
I think most battles would be over in about 60 seconds, even if they take 2-3 hours to play.
Does that imply that the rules are too complicated, or at least too time consuming?