I know that the preview on the Mac can take a second or two to generate but I have only used this with the newer intel Macs myself so what you are seeing could very well be due to the speed of your machine.
As for the misaligned preview is this the preview you see in the tool or afterwards when you see it in DW? Dreamweavers ability to display pages as browsers would is not always that great, you should be previewing in browser to see the most accurate view.