Its still slower, dual core or not. Technology has moved in favour of more efficient, higher cache chips in favour of clock speed. Theres not much point in going into it without wasting a couple of hours talking about architectures, processor efficiency, buses, reduced instruction sets, pipelines.
Take a look at this video from a Macworld keynote, it might help explain why clock speed isn't everything http://uk.youtube.com/watch?v=PKF9GOE2q38