Can you incrementally build the scene up, loading links 1-by-1, to isolate the culprit, to focus your troubleshooting?
Knowing the project, the models aren't built like-for-like, so whilst I also shudder at the thought of turning off door hardware, if you've MEP models loaded, there'd be my first go to to kill off sub-cat. junk. Likewise sub-nested .dwg content hidden in .rfa. All the usual bad stuff in Revit hurts Enscape considerably - but then, so do a lot of proxies.
I'd also echo Phil's thinking that render assets are playing a huge part - but more again in terms of parity between the models than mega texture maps.
Q. Is this for an entire site walkaround export? On sites that scale and style I'm surprised you're not going the teleported panaroma route.
In the end, there's always putting in a purchase order for that Titan.