If you let photoshop slice and then generate the HTML for you, it creates a huge mess of code and tables for layouts etc.
Photoshop is very good at what it does best - design stuff and thats where it should end.
Using the crop tool to get the elements that you require is a much easier way to get only the parts you need.
not so much speed, <table> is for tabular data - not layout purposes.
Yes, the faster internet speeds mean that images can be larger but there are still some people on 56k dialup modems!
Using the crop tool to crop around the images you need for the nav bar and then saving them. You then write the HTML in a semantic and correct fashion.