How can HTML assets be download before HTML content has finished dowloading

I am trying to improve my site load speed. When checking on the network requests, there is something that doesn't make sense to me:

How can the content that is going to be requested (css and js files) is being download before the html content has been downloaded? I have tried with a hard reload and empty cache from chrome, but this has happened again. Morover, the files seem to be dowload from the server and not from the cache.

Another thing I don't get, is why chrome is pating the bar chart almost all with blue, when the majority of the time is waiting for the server to respond (TTFB).

Thanks in advance!