Earlier quoted context omitted.
It took almost a second for me; I used devtools and it was about 200ms total for connection/TLS/download of the HTML (which is in fact under 14k (a bit over 35k uncompressed)). But then none of the assets (including the stylesheet) were queued for download until after 900ms. Does it really take 700ms to decompress and parse ~35k of HTML?
Site owner here! Even if it did take that long, browsers are clever enough not to wait until all the HTML is fetched as HTML can be processed in a streaming fashion. You can see here that stylesheets https://www.webpagetest.org/customWaterfall.php?test=240913_... are requested from the first bunch of HTML. A second is little disappointing, but in fairness my lowly, single CPU, single core server with only 1 GB of RAM…
My question was more of a "WTF is Firefox doing in the ~700ms between when the HTML has finished a network transfer and before the .css file is requested?"
I just tried it today and things are, in general, much faster (presumably due to the reduced load), but there is still over 500ms between when the .html completes the download and the .css is queued for transfer.
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I just tried it in a private window and most of the delay disappears, so I suspect extensions.