Why X11 via unaccelerated driver is faster than via frame buffer driver?

[Alex <lausgans%gmail.com@localhost>]

Hi folks.

Does anybody knows why on (some?) old platforms, an unaccelerated*
dedicated graphics driver (for example xf86-video-suncg6) is faster
than non-shadowfb backed frame buffer device driver**?
And also still faster than shadowfb backed wsfb on some operations
(like 10x10 scroll / copy).
[Actually, on CG6 / TurboGX with wsfb+shadowfb I see a slowing sliding
effect on some x11perf tests, so I don't believe the numbers here,
perhaps the sync issue]

Thanks!

* NoAccel True
** wsfb, ShadowFB False. A trivial fix like
http://pastebin.com/8BqjHWys was done in order to make wsfb work over
cgsix console