Fundamentally, mechanically moving a mirror is slow, which puts a lower limit on shutter lag. Currently the imaging chip is made so that it takes a while to reset for the real exposure. But this can be changed, and has been improving.

The real difference is the resolution. With the optical view finder your resolution is limited by your eye, while the display on the camera will always be crappy. Looking at the real stuff is always better than looking at the crappy display.