Very slow "add to cart" button

[zaglebie]

I have problem with my store, its working fine in all elements except "add to cart" button

when i click it it takes ALWAYS about 3 seconds for checkout to load (in my store add to cart = adding to cart + redirect to checkout because people mostly buy only 1 item)

 

i'm attaching chrome developer result, what is strange for me

 

first i have 2 302 redirects, so temporary redirects, why ? but it is not the main cause

 

main cause its what is happening next, 2.70 seconds, in description  Waiting (TTFB) (time to first byte)

 

how to check what is causing that ? maybe some module is loading data, i've read that fedex module caused same thing because it connected to fedex API and store have to wait for answer, but im not using fedex module

so how to check which module or which element is causing such a delay ? mysql slow log query shows nothing

 

i would like to know what exacttly happens during this 2.70 seconds, what operations are performed