There are many things you can do to make Indigo render your scene faster, and cleaner, often without compromising quality.
The first thing you can do is make sure the scene you are rendering is suitable for the power your computer offers.
If there are many light layers used, it will consume a lot of memory. If this exhausts all physical memory the rendering will become extremely slow due to disk access. For more information on light layers, see this section of the manual.
A larger screen resolution, and larger Super Sample factor, will take longer to render, simple because there are more pixels to cover. See Imaging Settings
Check the recommended system requirements to see how your computer compares.
Indigo has several render modes, which control how it chooses where to render, and how. Which mode is appropriate to use varies from scene to scene. Scenes with a lot of specular materials, for example, should be rendered with MLT enabled.
See Render Mode.
Some things about the way the scene is made can slow a render down, and likewise speed it up.
If a colour (this includes any colours in texture maps) is over-saturated, it creates an unrealistic conservation of energy as light bounces off it. This can create strange artifacts known as 'fire-flies', along with other problems.
Make sure that no colour is more than 80% saturation, which translates to an RGB no higher than that of 204,204,204.
Glass and liquids are one of the things that make Indigo renders stand out. However they can be computationally intensive, and therefore it pays to be aware of how they can affect render times.
MLT is good for clearing glass and liquids, and combined with bi-dir it can help those otherwise hard-to-render areas
While it is important to model your scenes as they would appear in the real world to realistically render with Indigo, modeling a light bulb as glass around a light source can greatly lengthen render times. This is because every single ray of light that is emitted has to pass through the glass and reflect & refract its way through it. Effectively this means that all light in the scene is now a caustic. Simply giving the light-bulb's outer shape an emitting material will give the same effect as a real light-bulb, without the extra calculations.
This material is especially difficult to reproduce efficiently, because of the complex effects it creates. It is strongly suggested that this material not be used to transmit the sole light into a room (as a sky-light for example) See Glossy Transparent
This feature allows glass sheets/panes to be rendered extremely effectively. If the scene has sheets or panes of glass, enabling this will reduce your render times. See Render Settings