Bit serial made a lot of sense because the memory was often serial (drum memory or acoustic delay lines use in 1960s calculators).

>And yeah, I also wished DEC didn't kill the PDP-10 for the VAX, but that's another story.

The DEC-20 was 6x faster than VAX-11/780 on at least one benchmark (that I wrote when I had access to both in the 80s..). DEC-20 was a PDP-10 implemented in ECL, it was quite fast.