6 hours and 15 minutes? Surely not
When I played HM1 I remember that a lot of battles ended with me (GM) saying that "The enemies are to badly beaten and give up" because the battle became uninteresting and peoples minds has started to wander. The "bookkeeping" takes a lot longer, even if you are experienced, especially for the GM. But the battles also takes a lot longer, especially if the first minutes or so only results in hits that doesn't generate shock rolls, meaning only a penalty. This can (and it often did) end up with a group of PCs and some Garguns for example that are badly injured (EML of perhaps 5-10 after injuries) and a battle that will consist of a lot of misses until somone end up with a lucky strike (often the PCs if me as the GM rolls hidden ).
I remember one PC (a Khuzdul) that had 174 IP (HM1) before he went down (a lot of strikes on locations that doesn't generate shock rolls). Sure his movement was down to five foot and all his skills was using the "EML 05" rule.
Even though I am generally more of the fan of the HM1/HMG rules, I do like HM3 in someways for speed of play. I like the HM1 injury tables, but I also like HM3 Injury and Fatigue for easy calculation of penalties.
One thing I'd agree with though, is that HM1/G has too much of a "death spiral", EML's that dwindle to nothing with any entity that has a high END.
HM3 though IMO is a little to fast with combat - only meaning that the penalties due to injury result in too quick of KO's. If I have a 2IL foot wound, I roll 2D6 in addition to any other injury penaties I have, plus the injury that actually requires a sock roll. If playing HM3, I'd go one added to the roll for every IL or FL. Actually, I think this would make more sense for HM1 also - So a dwarf with 18 END who is reduced to a -50 by injuries/fatigue is adding 10 to any shock rolls, not addign 5. That is a big difference - From a 13 on a shock roll being needed for a KO to an 8 being needed for a KO.
The one thing that I think might be realistic - "morale" rolls from time to time based on injuries taken in combat. And this morale may mean flee, drop to the ground, or even just to be totally defensive - I think that is one reason why HM1/G combats can be so drawn out, and deadly where 75% of the contestants are dead after a melee.
A roll against Initiative perhaps, Modified by any IP/IL's sustained. And maybe you roll this whenever you take a 3IL injury or better, or if you are stunned. IDK, a never playtested it.
And while it would add more rolls, these additional rolls would serve to get rid of contestants and actually make combat quicker. I think all the various incarnations of HM have rules similar to this for morale.