I couldn't find any posts containing the fix instructions when I wrote mine, then I had to leave the PC for a while. Back now, managed to find the quote.

Quote Originally Posted by 2KGreg
Hey everyone,

As you may have seen in other threads, the guys at Firaxis have a suggestion for the people who have been experiencing issues with the latest patch (early game crashes as well as slow turns).

Please follow these directions and see if it helps out on your machine, and report here to let us know!

1. Open up your config.ini (My Documents/My Games/Sid Meier's Civilization V/)
2. Look for the following line: EnableGameCoreThreading = 1
3. Change it to: EnableGameCoreThreading = 0
When that setting is on (default) multicore PCs will only use a single core. Some people find that leaving the setting on improves performance, many seem to find it makes no difference, and others find that it absolutely murders performance. My i7 2600k was taking longer to process the very first turn of a brand new game than it used to do to process a late-game turn with a very large, busy map. Some people also find that the setting affects game stability; I didn't find it made any difference.

This change rolled out as part of the most recent standard Civ 5 patch. All game owners will have it whether they own G&K or not.