One reason tables slow down a website is that the entire table is loaded before anything is displayed. The table needs to calculate everything before it can display the content. Each cell's measurements of width and height are figured out first, because each cell has an affect on the other ones.
Say a table has three cells with widths of 25%, 50% and what ever is left. The computer does not know where to put the second cell till it has drawn the first cell. Also the width of the last cell is unknown until the widths of the other cells have been been calculated.
If the web designer put a cell width of blank, 50% and 50% then it is easy to see that the first cell cannot be drawn till all the cells have been laid out.

I like divs myself because they can be displayed right away, and do not have to wait for other items. This makes a difference especially with dial up users.