For to designing a web page, keep in mind that nested table issue. Because, A single table on a web page will not slow downloading. But you put more tables nested inside another table means, the browser gets more complicated to render that page, as a result the downloading of that page will slow.

Normally when a page loads, the browser looks for the start of the HTML and it loads the sequentially contents down the page. However with nested tables, it has to find the end of the table before displaying the contents present in that table to the browser.

If you design the page with more tables will load lot more quickly than nested table design in a web page. Let us consider one example,

HTML Code:

<table border="0" width="100%"><tr><td>Header Content</td><td>Sub Contents</td><td><table border="0" width="100%"><tr><td>Other Informatiive Contents</td></tr></table></td></tr></table>

The Above Piece of Code can be written as follows....

HTML Code:

<table border="0" width="100%"><tr><td>Header Content</td><td>Sub Contents</td></tr></table><table border="0" width="100%"><tr><td>Other Informative Contents</td></tr></table>

This page will load quickly than the earlier page. Because, the browser render the first table and then the second one.

For additional Information about the efficient designing of table, refer this url
http://www.discussweb.com/web-design-help/3182-website-performance-tips-tricks-4.html#post9233