I started to migrate my widgets toward a more orthodox architecture and it’s a success.
The first heavy widget i migrated displays itself immediately instead of the usual 1-2s.
I think the problem was in my super smart organization which finally reveals itself completely stupid 
Not sure but i think my setup functions inside the hud postinit function were being called every frame, so i was basically recomputing the entire architecture/content of the UI each frame. Not a good idea.
Moral of the story: stick to the standards 
I also have greatly symplified my calls to the styles. All in all some great improvements.
Thanks again Szyszek for taking the time to investigate, forcing me to rethink my code and pointing me to the right direction.
Cheers
Cedric