I don't have enough experience with DDR5 to know much about the properties of the various ICs, PCBs, and binnings, but the performance sweet spot seems to be around 5200MT/s. Even mid-range boards support DDR5-6400+.
I've noticed the dip in performance when powering up settlements. It's quite significant, even without fires.
Anything refering to CPU or memory performance is refering to non-GPU-limited scenarios, which in my case, is anything up to and including 1440p ultra (at those settings my 6800 XT tend to bounce around 40-70% utilization in a high intensity CZ) that features populated settlements.
I went from this:
To this:
And gained a solid ~5% to average frame rates in the process, with a more noticeable improvement at the low-end. It was about the same difference as 400MHz+ of CPU clock speed on a Vermeer (Zen 3) part, which is a considerably larger differential than you'll see in the gaming clocks between, say, a 5600X and a 5950X.
The game still scales well with CPU clock increases, but improving memory performance seems to overtake CPU clock in importance after about 4.4GHz, on this platform. And this is all at the same FCLK, UCLK, and MEMCLK. The difference is correspondingly larger with a larger difference in the performance of the memory subsystem.
Going from some cheap 2400-3200MT/s, for example, to even budget 3600 stuff, then spending a little effort tuning it, would probably be one of the more economical upgrades that could be done for this game on AM4, if one already has a Vermeer. I suspect the situation is similar for any system that isn't GPU limited where a reasonably fast CPU has been paired with mediocre memory.