Any ideas as to why or how to speed up the screen painting?  All controls that have a ThemeSupport property are set to Full.

Welcome to the age old problem of WinForms   Actually there are a number of things that you can do.  First, in the 1.6.5 beta, all parent Themed Controls such as a ThemedContainer now have a property called RenderAsSolidBackground (or something along those lines depending on the control).  The reason things tend to render slow is transparency...which by default, all of the themed controls such as the ThemedGroupBox support this transparency.  The first thing that will increase performance is to prevent the reliance on transparency as this requires a re-render everytime each parent container redraws and there are MANY more rendering cycles that take place with transparency.  So on a ThemedGroupBox, set the TransparentBackground property to False and the FillBody to False as well (the FillBody is not related to transparency, however, it can improve rendering for controls that are dropped on this container).

I have created a quick sample here to show how to set the ThemedPanel and the ThemedGroupBox properties.  This is what we do in our medical software (along with some offscreen rendering) to improve performance on these large forms.  However, using solid backgrounds and preventing transparency make the biggest difference.

You can also lock handles in certain conditions as well...however, this requires more hand coding and should only be used on the worst of scenarios:

and

How do I make the ThemedContainer have a solid background?

Well, at the moment, you don't.  I will have to add this to my "to-do" list as this control doesn't yet have that functionality.  Sorry. 

I was already having to work on that control so I added two properties to it that will be included in the next build: TransparentBackground and RenderBodyAsSolidColor.  This will help improve performance like the other controls.