I've tried this numerous times, always the same. This was a simple scene.

If your system is using resource to refresh your screen, rendering large complex files can demand almost every bit of resource it can find.

While watching the white line trying to cross complex objects, it slows down. Yes it's processing the information in the file, however it's also having to "redraw" to your display at the same time.

While rendering to disk. you can notice the same slow down when it reaches that complex point in the scene, but it's able to utilize more system resource that's not being used for GUI-Display refresh.

Bryce 5.5

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Ockham's razor- It's that simple