Indeed,
Starting at a certain point:
Render time is proportional to item count.(in view)
Same with hatches.(Not strickly in view)
See: [SOLVED] Failed moving/isolating hatches disappear=missing
1.187.565 shapes making up the patterns in all hatches. 
Speed can also decrease a lot after an AutoSave.
dwg file size vs dxf backup.
About the fading: Did you try Screen-based with your last trial?
I find that the 1080 screen example is still with very thin lines. 
CVH