I am trying to optimize the following page: https://123tinki.com/nl-nl/c/geperste-hondenbrokken
It is served by an NGINX reverse proxy. Static content like .js and .css is cached by the NGINX server. The first ~30 recources seem to download fine. They have a TTFB of ~20 ms. But then suddenly TTFB rised to ~500 ms. If I load the resources individually in a new window, they have a normal TTFB. I checked the response headers on the resources and they are served from the NGINX cache (x-proxy-cache: HIT).
Why is the TTFB so slow? Maximum parallel downloads does not seem to cover the issue. In that case I would assume the download would start after the previous downloads finished. How can I investigate?