I have just realised how the speed of shade mode can be improved easily by switching to No light mode. The only thing with this setting is that all materials will be darker than the original texture. I think in no light mode all texture should be represented its original tone but in v6 it looks better. When I switch to default light the full viewport will slow down by 5 times but there is still a big help of the “Display object’s bounding box”.
I also found in v6 when I zoom close to the object, rotation will be hard but when I zoom out everything is smooth. The feel during the rotation is now more natural than in v5 I very like that but the speed should be smoother when I zoom close to a surface.