The Optimus is 600MHz vs 1GHz for the Nexus S. Unless I'm missing something, the Optimus has a normal ARM processor, single core, and just one of them.

The onscreen keyboard on the Optimus should be pretty much like any other Android phone. Play with another one and see how it works for you. I thought I'd hate the onscreen keyboards, too, but, other than every once in a while when I can't seem to type anything to save my life, I've been pretty pleased with the normal Android keyboard.