It seams to be caused by PoseDot. (GearHelper.lua, called in GH_Events --> BagUpdate + PlayerEnteringWorld).

Add script to each ContainerFrame(1->5):IsVisible() to call PoseDot only when the frame is visible and not at each loading screen ?

https://us.battle.net/forums/en/wow/topic/20753406958

NewWeightCalculation seams to add 11s at login
GetStatsFromTemplate seams to add 13s at login

event BAG_UPDATE is fired 200 times when /camp and 400 times on first load (or after a /quit)
It call poseDot which execute NewWeightCalculation for each object so for 100 objects it means 20K call for each loading screen
As NewWeightCalculation call GetItemInfo and it may take sometime 1 sec to get data from server, it add a lot of time at each loading screen

@Marsgames is able to reproduce the issue
On my side i'm not able