The complexity and size of a color or pattern affect render speed. More complex patterns require the rendering engine to calculate more samples so it can capture the details. The rendering engine works best when it can identify areas of similar surface treatment and can estimate appearances over large homogeneous areas.
For example, a smooth monochrome surface renders more quickly than a smooth patterned surface. A large-scale pattern renders more quickly than a dense, intricate pattern. A detailed, perforated surface renders more slowly than a simple surface.
The material render appearances that require the most time to render are (from slower to slowest): metallic paint, flecked metal, hammered metal, water, frosted glass, and perforated metal. Slower render times for these materials are proportional to how much of the scene they cover.
At the draft to medium quality settings, complicated materials may show many artifacts (small inaccuracies or imperfections in the rendered image). Imperfectly reflective materials (such as wood floors and metal mullions) appear speckled. These issues can be improved by adjusting the Blurred Reflection Precision value. (See Render Quality Settings.)
To improve the appearance of patterned surfaces and silhouettes without significantly increasing render time, adjust the Image Precision (Antialiasing) value. (See Render Quality Settings.) To produce images with little lighting depth but crisp geometry, use the draft quality setting with a high value for Image Precision (Antialiasing), such as 6.