[vtkusers] Inconsistent rendering times

[John Biddiscombe]

>pipeline and then update the screen, it sometimes takes a few minutes to
>get out of the Render() call, while other times it takes only a few

when using an imagemapper ...
if the data is in unsigned char format, the pointer can be passed directly 
to glDrawPixels, if the data is float, or other, it has to be scaled and 
wotnot. This causes a delay.
Also, if a slice is contigous in memory (usually a zslice), then it can be 
rendered directly, but if taking a slice on another axis, it has to be 
extracted and passed.
These two are you main delays, but may not be valid in your case....

JB