KEYSHOT: Turn off NURBS Modeling under Render

• parts that disappear (or are hidden) from screen

Turn off NURBS Modeling under Render menu to see parts that disappear (or are hidden) from screen.
Right click screen to show all parts.

• why keyshot so slow?

Are you working with high-poly count models? Working with NURBs turned on? Loading heavy resolution texture maps? Ray-bounces?
Also, the advice I've always been given is to turn down the processing power Keyshot uses for real-time rendering while you're working. The default is 100%
You can usually get away with turning down 1 core or 2 in the pull down menu towards the upper left portion of the screen.
(https://forum.keyshot.com/index.php?topic=21436.0)

• use actual dpi of the image which allows you to maintain the proper size:

The following video explains how to use the DPI settings:

http://youtu.be/xlA5HhTy0-I