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Render the frame at half, quarter or eight of the original resolution. Speed gains are proportional to the square of the resolution decrease. For example, half the resolution will produce frames four times quicker.
Sampling
Use only a fraction (100%, 25%, 10%, 4% or 1%) of the rendering settings Shading Samplesthe Shading Samples specified in the render settings for quality. Speed gains are linear with the samples decrease: half the samples usually means twice the speed.
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