I don’t know if you’ve tried this or not, but if you stop the rendering at a certain frame and then after launching again you find that the rendering time has returned to normal, then there must be another problem besides what I mentioned. If you’re using GI, so as some users have advised, you could set the settings as follows :
Render Settings/Global Illumination/Cache Files – Prepass only:off – Skip Prepass:off – Auto Load:on – Auto Save:on – Full Animation Mode:on
If this isn’t the case for you, so you could copy the geometry into a new file and use “Current state to object” so that you won’t have any effectors. such as Cloner, Sweep … etc, then try again. I hope one of the two things will work for you.