In Unity 5, the lighting is being processed automatically in the background after you've changed anything in the scene that could affect the final result. It's a very neat and interesting feature that works surprisingly fast on modern computers. However, despite its fantastic optimization, it is still noticeable, especially when you are working with reflections that take much longer time to be calculated compared to lights. To enable calculation on demand, do the following:

Now, if you need to calculate the lighting, you can click on the Build button next to it and execute the process on demand.

When you disable the Continuous Baking feature and rely on build on demand with the Build button, Unity will be taking a snapshot of the level's lightmap, create a folder that has the same name as the level and store the snapshot there.