First, your server seems a little sluggish.
Apart from the server being slow in general, there are a few things you can do to help cut down the work Zen Cart does in calculating things for your site:
There are already a lot of calculations being done to display your cart contents. Doing it repeatedly for several products takes time, and that time increases with more products in the cart. In v2.0 of Zen Cart, a lot of work is being done to make these calculations more efficient. In the meantime you will have some increased slowness with larger carts.
To minimize a lot of the slowness on the shopping cart page, turn off the** shopping-cart sidebox** when you're on the shopping-cart page: Admin->Configuration->Layout Settings->Shopping Cart Box Status --- SET TO 2
Shopping Cart Box Status
Shopping Cart Shows when:
0= Always
1= Only when full
2= Only when full but not when viewing the Shopping Cart
Also, in Admin->Configuration->Attribute Settings, there are some settings you can turn off if you're not using them, thus improving speed when attributes are used:
Enable Price Factor
Enable Qty Price Discount
Enable Attribute Images
Enable Text Pricing by word or letter
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