How to identify and reduce first byte time

Web Performance Discuss Test Results
1

We have a custom ecommerce site that received an “A” for first time byte on the home page, but is getting an “F” on the pages that dynamically display content. I read through the documentation, but I’m having trouble identifying what to fix. How can I fix our site’s performance?

Test results
http://www.webpagetest.org/result/151124_BC_KYZ/1/details/

Site

2

One thing that’s puzzling is how long it takes for the drop-down, light blue arrows to appear in the top blue horizontal navigation. I’m trying to identify the bottleneck.

3

The standard ‘combine css and js’ to reduct the file count applies in general on the site, but that doesn’t affect TTFB.

Other than that I can’t really help as I only speak linux.