I modeled a house and wanted to render the rooms from the inside with the approximate sun lighting through the window. However cycles is incredibly slow in doing that. What should I change to increase my render speed? I am okay with sacrificing quality for it as it is not yet my final render and I am still planning the room layout.
The room is physically correct with ceiling and floor (cube with multiple small holes in it). I'm rendering in perspective mode using Cycles.
It sounds like you are using a sun lamp for it, that really slows it down. So the only thing I can add to Gwenn answer is: for interior renders, use planes with an emission shader in every window hole, and disable the "camera" option in the "ray visibility" panel in the object properties so you don't see it when rendering. You can combine that with a sun lamp if you want hard edge shadows coming from the sun.
I use this method a lot and it really speeds things up.
There are a number of things you can do to reduce the render time in Cycles. Here are a few that I frequently use:
Reduce resolution: If you can get away with a smaller output image, or if you're just doing a test render, set the 'Percentage scale for screen resolution' to something low, like 25% or 50%.
Decrease the number of faces: On small items that don't need an excruciating detail, instead of using a subsurface division modifier, set the shading to 'smooth' and consider using a decimate modifier to reduce the number of surfaces that light has to reflect off of.
Switch to GPU rendering: In the User Preferences view, go to the System menu. Under 'Compute Device', select CUDA. Then, when you go to render, in the Device dropdown, you'll have the option 'GPU Compute'. (This only works if your computer has a GPU with a CUDA compute rating of 2.0 or higher.)
Decrease the number of light bounces: If you don't have highly refractive materials, like glass or gemstones, it can pay to reduce the number of light bounces Cycles computes. In the 'Render' menu of the Properties panel, scroll down to 'Light Paths.' If you set the minimum number of bounces to 0 and the max to a low number (like 4 or 5), you should greatly improve your render time without sacrificing too much realism.
Change tile size: In the Properties panel > Render menu, scroll down to 'Performance'. If you're using GPU rendering (see above), set the tile size to 128x128 or so. If you're using CPU, set it to 16x16.
Simplify: In the Properties panel > Scene menu, scroll down and check the Simplify option. This sets global limits on mesh detail size and soft shadow calculation that can really improve rendering time.
Use 'Multiple Importance Sample': For each of your light sources, scroll down to 'Settings' and select 'Multiple Importance Sample'. This greatly speeds up the rendering time for glossy and diffuse surfaces, and allows you to use fewer light samples to achieve the same quality render.
One thing that may speed it up faster than anything else is lowering the bounces. Since the image is interior, you'll have a lot of light bounces greatly increasing render times. To fix this, try setting the min bounce to 3 and max bounce to 4. You won't loose too much realism and if your iterating and rendering a lot, it helps to speed things up.