I've seen a few discussions about the cause of FPS death. Can you please explain in simple terms exactly where the bottleneck lies, and how easy or difficult it will be to address?
Honestly, my impression of Toady's development process is that the answer to this question is "mostly no". Toady addresses FPS problems as bugs, not as metrics. There have been fixes that address FPS issues, and over times where Toady has chosen to only partially fix issues, so those amount to known bottlenecks. In the same vein there are some subsystems that are known to be slow, such as fluids.
Additionally, there is anecdotal and player evidence that certain things contribute to FPS problems, with mitigating effects. You've probably seen these in discussions, and I believe there's a wiki page on these and what can be done about them.
Finally, we can talk broadly about how DF uses memory. This helps answer questions like what kind of hardware is best for DF and will different methods of parallelism help. Dwarf Fortress uses a lot of memory, for accesses a lot of it every frame. This is anecdotal too, since I'm not aware of anyone actually profiling DF.
A proper answer to this question would require metrics, and those simply don't exist.