Well, AFAIK DoC has even longer wait times between turns in the lategame. SoI is in a great position in this regard - while the map size is more or less the same as in RFCE, it has much less room for cities and units, thus significantly faster in the lategame. Keep going? The mod didn't get any slower (at least not significantly) in neither version 1.4 nor 1.5. The last bigger drop in performance was in 1.3, when 6 new civs were added, as adding those also resulted in a much higher map density. Also, wasn't it clear already that I try to be as careful with bigger additions as possible? Just check back how this conversations started: you suggested to increase turn number to a much greater extent, and I said I don't think we should have more than 600. The point is, while I'm far from being a pro in this regard, I always try take performance questions into consideration too.