What do you lower to get the most bang for the buck performance/image quality?
It's odd, because I have older tech than you and my PC has no trouble between turns even in the late game on a large pangea, with low sea levels and 50% more civs and CSs than standard for "large".
However, I did have an issue the other day that shed some light on things. For some reason my overclock failed on boot-up, but I didn't notice and when I went to play Civ5 I thought it seemed to be really slow. I checked my overclock and sure enough I was back to stock. After I re-clocked to 3.6Ghz (from 2.4Ghz) I was back to quick in-between turn times.
So, my suggestion is to overclock your CPU some more. I don't know much about the "i" architecture of your CPU vs the Core2 of mine, but I'm guessing yours is probably better clock-for-clock, so I don't know how much overclocking more will help, but give it a go. I'd posit that Civ5 was coded up on/for Core2 architecture, so might be better suited to it than the later "i" architecture, but more Ghz can't hurt, so give it a try.
Also, I've seen a lot of complaints about slow in-between turn times from Core "i" users. Perhaps it's the frequency scaling tech that is messing with the performance? IE, on your turn, the CPU isn't doing much, so scales back the GHz, but when you hit "end turn" it has to crunch a lot of numbers and maybe doesn't respond to Civ5 as it should? I don't know for sure, but maybe it's a possibility? Try turning off frequency scaling in BIOS (if there's an option) so your CPU is always at a fixed, maximum frequency and see if that helps.