So a bunch of the technology was done on the
assumption that power
density could be very high and speed would be the maximum possible at
that time. Also, I think it was the only VAX ever shipped out of the
"large systems group" which used to be the PDP-10 group before the
last PDP-10 project got nuked halfway through its development.
Again I googled a while ago and I learned that the "large system
group" delivered VAX8600 in between PDP-10 and VAX 9000.
paul