why my site is so slow ??

Collin,
If you go to Admin->Configuration->Logging, and choose "DISPLAY the page parse time" ("display", NOT "store"), and set it to true, it will show the speeds in your footer (assuming your custom template hasn't deleted the code that does that). What counts are you getting on your various pages?

Do you use the "featured products" facility? If you turn those off, does it change the speed?
Do many of your products have sales/specials? If you turn those off, does it change the speed?

Do many of your products have attributes? Are many of those products set to "priced by attributes"? If none are, then turn off the Enable Price Factor in Admin->Configuration->Attribute Settings.
Do you have any products that are "priced by words" or by "letters"? If not, turn that off too.
Same with downloads ... if you have no downloadable products, turn that off.
Turning off those things will stop the store from going through pointless calculation checks.

I suspect you'll find that your server's database is the bottleneck, though.

before turning off attributes
Parse Time: 7.479 - Number of Queries: 260 - Query Time: 0.17374306993103 (main page)

Parse Time: 6.783 - Number of Queries: 805 - Query Time: 0.75481033467102 (acer laptop page)

Parse Time: 6.066 - Number of Queries: 606 - Query Time: 0.44305406065369 (canon digi cam page)

after turning off attributes
Parse Time: 1.823 - Number of Queries: 261 - Query Time: 0.11577211100769 (main page)

Parse Time: 2.554 - Number of Queries: 805 - Query Time: 0.56980977964783 (acer laptop page)

Parse Time: 1.332 - Number of Queries: 605 - Query Time: 0.2527602447052 (canon digi cam page)

the featured and sales products made no difference but the attribute changes helped alot thanks for your advice

Just for information.

My "product_info" page is displaying very slowly on some products.
This happens when the product has many attributes values.
In fact the "problem" is coming from the file "modules/attributes.php" :
line: $new_attributes_price = zen_get_attributes_price_final($products_options->fields["products_attributes_id"], 1, '', 'false');

100 values in a dropdown attribute takes almost 4 seconds of processing!.

zencart v1.3.8