Exactly what Tom said, or start a new empty scene, then start merging your objects one by one (or first half, then half of the remaining objects, and so on until the slowdown appears).
But since you mentioned the slowdown appears after merging a single object ("after merging an object from another file, rendering has slowed to a crawl") - then I guess that object is the culprit and the remaining part of the investigation is to figure out why it's slowing down.
Is that object related to some 3rd party plugin?
Is it using a complex material and/or a material using 3rd party plugins?
It could be a bit of column A and a bit of column B.
It has a railclone object in it thats gernerating lots of lights. However, the same number of lights when created manually doesnt cause the slowdown it would seem.
So i think it could be railclone, lights and material complexity coming together into a perfect storm.
I think ill put ijn some hours this evening and strip back the file.
One thing i have noticed though is that even when i hide the imported objects, the slow rendertime stays there.
If i dont import it, the scene renders fine.