Initializing in cycles taking AGESi


I have been working on a project for several months now and the entire time I was able to render with little trouble. Suddenly, perhaps because of an attempt to update Blender which I then reversed, the ‘initializing’ step in Cycles is taking up to 5 minutes per frame. Since I have 656 frames, I can’t render my project without leaving my laptop working for several days.

Why might this have happened, and is there any way that I can undo it?