In support of this, it would seem that the access the vga chip
affords to memory through the bus is inversely proportional
to the speed at which it can bitblt itself.

In other words, the more the chip does for you, the less the
designers feel the need to put in decent bus access.

What really ought to limit it is the priority access the playout
has. So, it you want to speed up your vga i/o, perhaps turning
down the refresh rate is the answer. Somehow I doubt it.