Http Get Request Breakdown

Developers Server & Performance
1

When I click on a product on a home page under best seller, it generates HTTP GET requests like this:

GET "http://example.com/category1/subcategory1/product1/HTTP/1.1"200 22559 "http://example.com/"
GET "http://example.com/images/thumbnails/500/350/detailed2/product1_pic1.pngHTTP/1.1" 200 223243 "Product URL"
GET "http://example.com/images/thumbnails/500/350/detailed2/product1_pic2.pngHTTP/1.1" 200 148742 "Product URL"
GET "http://example.com/images/thumbnails/40/40/detailed2/product1_pic1.pngHTTP/1.1" 200 2532 "Product URL"
GET "http://example.com/images/thumbnails/40/40/detailed2/product1_pic2.pngHTTP/1.1" 200 2172 "Product URL"
Any idea what exactly happens behind the scene between first GET request and second GET request for product image?
Reason I am asking is I occasionally it takes minute or minute and a half between first and second request. On the front end I get "waiting for response" message and after a minute (after GET requests for images) product is displayed. It happens randomly not every time. Hosting staff can't find anything out of ordinary, site has no traffic and 10 products so there is no load or resources issue.
I am guessing mod_security is causing the problem but can't figure out how to capture the issue to understand it. Any ideas or help would be appreciated.
2

When I click on a product on a home page under best seller, it generates HTTP GET requests like this:

Any idea what exactly happens behind the scene between first GET request and second GET request for product image?
Reason I am asking is I occasionally it takes minute or minute and a half between first and second request. On the front end I get "waiting for response" message and after a minute (after GET requests for images) product is displayed. It happens randomly not every time. Hosting staff can't find anything out of ordinary, site has no traffic and 10 products so there is no load or resources issue.
I am guessing mod_security is causing the problem but can't figure out how to capture the issue to understand it. Any ideas or help would be appreciated. 

GET "http://example.com/category1/subcategory1/product1/HTTP/1.1" 200 22559 "http://example.com/"
GET "http://example.com/images/thumbnails/500/350/detailed2/product1_pic1.png HTTP/1.1" 200 223243 "Product URL"
GET "http://example.com/images/thumbnails/500/350/detailed2/product1_pic2.png HTTP/1.1" 200 148742 "Product URL"
GET "http://example.com/images/thumbnails/40/40/detailed2/product1_pic1.png HTTP/1.1" 200 2532 "Product URL"
GET "http://example.com/images/thumbnails/40/40/detailed2/product1_pic2.png HTTP/1.1" 200 2172 "Product URL"