You need to avoid using spherical lights with emitters assigned to them. Try making your own lamp object rather than using the ArchiCAD lamps as they often use hi poly objects like spheres to make the bulbs. Maxwell is very slow with interior renderings and you can help yourself alot with noise by making the emitter a simple 4 sided plane nested up in the can with the down surface as the emitter only.
Generally speaking, these tips can help you.
1. DO NOT TRY TO MAKE THE INSIDE OF THE CAN A HIGHLY REFLECTIVE MATERIAL. This will add time to the render as well as noise that may never clear up.
2. DO NOT TRY TO MAKE A TRANSLUCENT SHADE TO COVER YOUR BULB. This will introduce noise that will never clear. It is better to create a .mxm material in the Maxwell Material Editor and use a .mxi image as a lightsource that is mapped directly to the lamp shade or cover to your fixture. It can look more photographic and will not intorduce the noise AND will render much faster.
3. For white walls, DO NOT MAKE YOUR WHITE HIGHER THAN RGB 240,240,240. In fact, it is suggested that no color should ever exceed 240 as the physical rules of conservation of energy are better observed. Higher values reflect more light than realworld objects generally do.
4. Make MXMs for your lights. Assign them in the ArchiCAD materials editor so they render. Making MXM's allows you to choose accurate color temperature settings that the plug in doesn't.
5. Using Multilight requires that you group archicad lights by assingning an AC Material to a set of light sources. They will export as a single object. Therfore you need to set the wattage to reflect the total number of lights in the new object. So, if you have (8) 50 watt MR-16 lamps in the ceiling, you should go make an MXM that is 400 watts with a color temperature of 3050 kelvin. Assign this to the simple plane in your lamp and you are good to go. The plane need only to have the emmiter material assigned to the bottom. Not the sides or top. It will save on calculations.
6. Avoid Displacement, Coatings and be careful to not over do it with high reflection values. These all have a taxing effect on the typical computer. Unless you have an 8 core machine, a very simple scene or access to a network rendering facility like
www.ranchcomputing.com you will be waiting for a day to see something decent.
7. Remember that photorealisitc renders do not mean photographic renders. Turning on all the lights in your scene shows the funtional lighting of the space, but rarely are all the lights on in a building. If a photographer properly shoots an interior there is a high probablility that flash and fill lights would be needed as well as maybe even turning off some building lights to get an aesthetically pleasing photograph. Be prepared to add some fill lights and play with the color of the lamps to get a pleasing result.