Very slow after 100 products in shoppingcart

[tukker]

I using 1.5.6 with theme http://www.prestahub.com/pres-3-theme.

 

When customers put over 100 products (stamps) in the shopping cart al is going very slow. 

Checkout is then a crime. Sometimes blankpages. In the loge file on directadmin I see this :  maximum execution time of 30 seconds exceeded

 

When so much products are in the shopping cart it also takes a long time tp put anotheren product in the shopping cart.

I sell stamps so it is very often that customers buy more then 100 products.

 

Prestashop 1.5.6. is running on a VPS with 2Gb memory!!

 

Can anybody help me to fix this problem. 

http://www.zegelstekoop.nl