Fjord weights a ton. And sags too.
Granted, I don't have a 45 Birkin in fjord, just a tiny 31 Bolide, but it is as light as my gulliver Bolide and lighter than my 32 and 35 box Kellys.
And excuse me! It's VERY stiff and hard! I cannot see it slouching, ever. (But that may be a result of the bag design more than the leather itself). I love fjord.