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Render time is reduced for overexposed scenes, by taking into account absolute light intensity for adaptive sampling. This can negatively affect some scenes where compositing or color management are used to make the scene much darker or lighter. For best results adjust the Film > Exposure setting to bring the intensity into a good range, and then do further compositing and color management on top of that. Note that this setting is different than color management exposure. Previously Cycles' adaptive sampling used sqrt(I) to normalize noise level to conform to a viewer's eye sensitivity. It is great for darker regions of the image, but also requests too much samples in bright regions, sometimes several times more than needed. Highlights can tolerate more noise because in most examples it is still less noticeable then the noise in darker areas in the same render. Differential Revision: https://developer.blender.org/D16392