First, thanks for all your efforts to solve this issue!
I made some tests based on your suggestions and I confirm everything you noticed:
1) deleting
all the CSS files brought the opening time to around 1 minute;
2) then, excluding any incorporated style (or probably because it's not the first opening...) brought it to some seconds.
Perfect!
As you mentioned in the discussion with poire-z probably the best would be to have an automatic tool cleaning unused, bloated or simply slowing down CSS...