The render appearance of a material specifies its reflectivity. Revit can render matte reflections quickly. However, any material characteristic that causes visual distortions (such as blurred reflections or transparencies) requires more work to render, and so requires more render time.
Shiny and mirrored reflections are slightly more difficult to render than matte reflections. A fritted surface is more difficult to render than a smooth, glossy surface. Water is more difficult to render than glass. Metal with a patina or a hammered surface is more difficult to render than polished metal.
Blurred reflections are the most difficult to calculate. However, you can control the quality of blurred reflections to reduce the impact on render performance. (Use the Reflections and Transparency options. See Render Quality Settings.)