I have an animation with an object that "grows" hair in the middle (animating the render->end->timing in the particle settings). It seems that the hair exists from the very beginning of the animation, and even during rendering of the frames before the hair appears, the "Building BVH" step in the render takes forever. Is there any way to make the render before the hair appears faster? Any help is appreciated. (I'm using cycles and blender 2.8, but I can probably convert an answer from 2.79 into 2.8)
Thank you to @FFeller for his comment, it really answers everything, but since he didn't write an actual answer, I will answer my own question for future viewers.
In order to disable a hair animation for part of a render to speed up BVH creation, as FFeller said in his comment, one would insert keyframes on either side of the period where the hair is present in pairs, with the enabled pair on the inside and the disabled pair on the outside (probably with one frame difference between enabled and disabled keyframes, otherwise results can be rather strange). This would be done by navigating to the correct frame, setting the camera icon to either enabled or disabled (depending on which is needed) and then pressing "I" over the icon to insert a keyframe. The icon should now be highlighted in yellow and there should be a keyframe on the corresponding frame.
keyframe which is set to enabled
keyframe which is set to disabled