If you want a simpler example, here’s one I just made:
1000_Cubes.zip (18.1 MB)
Use the view shown (Perspective ZEA), Make2D only that view, no tangent, hidden or scene silhouette, just Group checked.
Time to Make2D in V7: 1.3 - 1.35 sec.
Time to Make2D in V8: 3.8 - 4 sec.
3x longer!
With hidden lines and scene silhouette checked:
Time to Make2D in V7: 4.7 - 4.8 sec.
Time to Make2D in V8: 12.5 - 13 sec.
~2.7x longer…
It’s pretty much the case with anything you throw at it in V8.
Screenshots:
V7 - scene silhouette is incomplete, plus artifacts here and there
V8 - scene silhouette same more or less
But with a bunch of errors in the main visible curves 
All in all pretty much true to the title of the thread…