To answer the OP's question, we first must assume that the only difference between the i5 and i7 is the clock (i.e. GHz). In reality, the i7 has extra processing features that the i5 doesn't, but this shouldn't be too relevant to Civ, and this assumption give way to the following simplification:
The mentioned dual-core i5 effectively yields 5.2 GHz, and the i7 6.0 GHz. Therefore, the i7 is 15 percent faster than the i5 (5.2 + (5.2 * .15) = 6.0).
If in the lategame you're waiting 30 seconds between turns on the i5, you'd instead wait about 26.5 seconds on the i7. Is that worth it? ...
You must also bear in mind that you're not just paying for the improved processor in the more expensive computer; likely, the retailer is throwing in stuff like more RAM, bigger HDD, etc., which on top of the more expensive processor, makes the laptop significantly more expensive.
It doesn't help your decision that Intel has made several different i5s and i7s. There is the Sandy Bridge, Ivy Bridge, Haswell ... (
http://en.wikipedia.org/wiki/List_of_Intel_CPU_microarchitectures ). Some may have a more positive effect on your playing experience than others.
Now that I have bored you, let me get to what you're really after
:
If turn time is becoming too much for you in Civ, what you really need is a
quad-core processor. This almost
halves the time between turns compared to the dual-core. I play on a Sandy Bridge quad-core 3.3 GHz i5. Never timed the wait between turns, but it is fast enough that you don't feel like you're sitting around just waiting for your turn. There is still a noticeable delay in the lategame, particularly if you like to play with a lot of AIs and city states, but it is not unbearable at all.
Having a faster processor is the only way to speed up Civ between turns.
Unfortunately, unless you're building the computer yourself or otherwise have full customization over it, you'll be paying for all these extras on top of the processor you don't need. Computer manufacturers do that; they don't sell 10 different items in a line of computers, each only an incremental improvement over the other. Rather, in a line of laptops or PCs, there is usually a budget version, a middle-of-the-line version, and an enthusiast version. Not a "I just want the fastest processor you got don't care if everything else is garbage" version.
You can also look into overclocking the processor you already have, but this is not a good idea if we're still talking about laptops, which have overheating issues. Hope this all helps.
