The boost algorithm is designed to boost when until any of the following conditions are met; Temperature limit, power limit, voltage/clock limit, current limit or frame rate cap. (I feel as if I am missing a few) When you are viewing an area that is heavier on the GPU and more difficult for it to render it will require more resources to render each frame. Hence why the frame rate drops. The boosting system is designed to have the clock speeds as high as possible before reaching these conditions, when you go into a more difficult area to render these conditions stay the same and are reached earlier by the gpu. The boost algorithm is still working when you are in these more difficult areas as it is in the areas where you can get 60fps it just has more to render meaning the FPS drops. If you want to be able to hold 60fps consistently look into overclocking or undervolting.