Do you have the latest patch installed? In general, the game's optimization isn't great, and there are unofficial patch-fixes available on various websites. However, I won't provide links because I haven't checked them and can't guarantee they don't contain viruses.
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By the way, I just noticed the topic title... If the problem is only in the main menu, check the game itself, as the main menu runs significantly worse. That is, with your configuration, there's a high chance that the game itself won't lag; it's not as demanding.