I can only compare using the per minute costs of my laser vendor charged me for my pre GF engraves - I don’t know which machine he uses but I know its a 64 watt. His engrave charges are on par with the times I see on my GF - and frankly the results on the GF are cleaner even at 225 ppi. A laser that is engraving sooooo incredibly fast like a Universal sounds amazing - I wonder what it costs, or what a vendor would charge for the service?
Personally the plusses of visually lining up my artwork on any size medium combined with actually being able to afford a laser I can experiment with in my pajamas for exceeds the need for patience in engraving time. 
And like I said my experience with commercial vendors in the LA area are quite different from yours. I am always interested in learning about others experiences so I am not just in my own thought bubble.
Yes - to quote the vernacular of the day, if you increase zooms, you have to increase pews. If you don’t, it gets lighter. That’s one of the reasons we didn’t push harder on top speed; our current top speed is well matched to our top power. (Not that there isn’t some room for improvement, but for most purposes, it’s a good result).
This is also a limitation, and the reason that slower printing gives you higher horizontal resolution.
Looking at a really fast laser head zipping back and fourth, one thing that sticks out to me is the difference in mass between a simple open mirror and immobile lens and a head that has all of the systems the glowforge has.
Also, I have been amazed with the ability of the power supply to vary energy across a detailed topography at top speed.
I watched the glowforge engrave the profile of that diamond knurl pattern (thanks @mpipes) composed of lines of four sided pyramids. On a scale where there might be 15 of them in an inch, the power smoothly ramps up as the beam cuts to the deepest point and ramps down as it ‘climbs’ to the peak, sculpting four sharp flat sides that terminate in a point. I see nothing slow about the power slew in that example. It blows my mind.
I have seen video of other lasers move much faster, but again, there is a difference in reciprocating mass. I don’t think you could yank the weight of the glowforge head around that fast.
I assume focus-dot size matters a lot too? Since ultimately this is about energy density?
A separate factor. But to a certain amount, yes. However when cutting, you’re moving slowly enough that the heat moves out from the cut zone a bit, so tighter focus yields diminishing returns.