How sure are you that it's corona slowing down?
This kind of thing has been plaguing Cinema 4D for at least 10 years, usually, it boils down to some generator or sim. Do you have metaball or some kind of sim or dynamics going on?
Some years back I had something similar happen to me, renders started at around 5 min per frame and left it to render overnight, came back to it in the morning to find it still rendering with over 40min per frame now. In my case, it was a metaball causing the issue, and to solve it I simply exported it as an alembic first, so that it doesn't have to calculate every frame.
I've also had a couple of other cases with cloners that were solved in the same way.
When you start a render, where does the extra render time show up, in preparing the scene or in the rendering?