Slooooow add-to-cart

[cynamonstudio]

Hi, I've been looking for the solution with no luck - maybe you can help.

Add-to-cart action ( as well as removing products from the basket ) slows down the shop ... gradually - that means : the more products you have in the cart, the slower the shop gets. As you can see below , as more items land in the basket, the response time gets longer and longer, up to the point when the store is literally .... unusable ( here I am using the addon for adding products to the cart in a bulk to visualize better how things work now )...

BTW I am using 1.7.2 engine. 

Thanks for any advice/hint/help in advance!!!!