Basically any Python modding causes lag, because Python isn't compiled into machine code and has to be continuously interpreted in real-time by the computer/game.
Using some of the calls found in CvGameUtils would be done so often that they are disabled by default! Because making some Python call 100 or 1000 times within the turn of one single player is consuming huge amounts of processing power. So only enabling the callup - without even editing it - will cause lag. And the more code you add - the more lag will it cause.
Now, there can be more or less good or bad code. You could probably get around some of the additional lag caused by your Python mod with clever scripting, like caching techniques. But I'd rather write the code than try to explain how something like that works. The idea is however that you don't make the interpreter repeat some code when not really necessary, like if a city has already been checked for unit building it doesn't need to be checked again - on the same game turn.
But the only good solution would be to alter the SDK directly - then everything is pretty much immediate and there wouldn't be any measurable lag.
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that never works 
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