This one should be your first consideration. You could do a self comparison:
Make 2 same look pages, one using table, other using div. you should see the div version is lighter then the table version.
Graphic compression, will be your next consideration. You can optimize it using image ready or photoshop, or other graphic software. The save for web feature should be useful to lighten up your heavy page.

When you're done with the above, recompress your page using gzip compression.
you can read about it here: How To Optimize Your Site With GZIP Compression

Alrite, good luck.