When you use softshadows in normal mode they create messy shadows when there's any glass in the scene. When you render at premium the shadows are good again. The 20% speed increase seems strange to me. Did you render both renders directly after each other without shutting down bryce and changing anything to the scene? Bryce seems to keep some information when rendering wich can be used to optimize other renders of the same scene. That could mean some time-improvement. Other than that it's new to me that Premium is faster. I usually use Normal render mode, unless I want some premium options in my scene. I only use superfine when the textures are very noisy and need extensive AA.

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