It looks like the energy is out of balance in the scene, meaning, the material colors, the camera exposure, and possibly emitter & environment power values, are not being kept in realistic ranges.
The white material looks very bright; considering that the render engine is simulating how light really behaves in the scene, if surfaces are assigned materials that return more energy than they would in the real world, the light rays live too long, creating noise, and taking time & CPU. The R, G, and B values of a material color should generally be kept below the 230 range, since that is about the brightest value you are likely to encounter in reality.
Next, the camera should be set to use a realistic exposure; I would guess that to be around EV 11-12 in the daylight scenario. If you have to use something that seems unrealistic, it should be taken as an indicator that the energy is out of balance somewhere. You can find a chart of EV (Exposure Value) values for various situations on this page
Emitters should be set to use realistic values, even if this means you cannot see their contribution, since that is how it is with a real camera, as well -- you cannot expose both for weak and strong lighting simultaneously. And if lights cannot be seen contributing, it may be good to disable them, since they will still be calculated, and will therefore slow down the render.
Lastly, make sure that environment power values are set to defaults (i.e. 1.0); you can increase the sun and sky intensities, but that should only be used in situations where it is done for stylistic purposes. It is possible, though not very realistic, to decrease render time somewhat by increasing the sun radius, though this will soften the shadows it casts.
So please review these factors and see if they help.