I'm not worried about FPS death, at the moment. I play on a not-very-recent machine, often up the PopCap (200 to 250, IIRC, but something like that) and the inter-cavern spaces (from 5 to 20, again IIRC, but I know what I do when I'm configuring a new version's new Worldgen parameters), and so technically should be suffering more from FPS problems than someone running it vanilla, yet have never had to turn off temperature/run on 2x2 embarks/etc. Perhaps I'm used to a slower game, of course I remember a significant speed-up, perhaps between 40d and its successor, where suddenly everyone was running around much quicker, and perhaps I've now managed to make it work back down at a speed I can mentally handle.
I do get significant pauses in game-play when migrants (or others) wander on-embark, when I suppose the quantity and quality of the new arrivals is being 'rolled up', but that doesn't matter too much (and can be a nice indicator of something "about to happen"). Pauses in Minecraft (I've only played up to 1.8beta, no idea about newer) are a lot more awkward, especially when precision pouring magma or dealing with a hostile of some kind. If I was every playing multiplayer, I could imagine it being even worse, but I've never done that.
Aside from this, there's the memory loading. I'm rather of the school of thought that switching to 64-bit does (in a like-for-like background engine and a benchmark amount of data) actually use more memory than the 32-bit one, so that might be the problem. Of course, if values that are 64-bits in size (or can be better handled, that way) are in with the data, then you've got to make the move. But if Toady can keep things 32-bit and doesn't need to change it, I can see it working on our more archaic architectures still for a good while, and the bloat not be too unmanageable at all.
But, we shall see.
(Still, the way I play it's mostly on pause while micromanaging, anyway, so who knows if I'd notice lag.)
((Oh, and I just came across this thread, and wasn't aware of the other thread (unless I'd posted in there but it's really old and now inactive), and not yet checked out that link, my ninja-friend. I'll look at it later, but right this moment just post this here.))