I think this is no hardware issue, but shadow settings if wires shadows* are not on ground as you need.
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Any computer is enough for “baking” games with large maps like this. You have to wait, you can upgrade (not big difference, you will wait hours anyway), or on large level use dynamic lighting only, preferred solution. With correct shadow/lights settings you can have better effect than with baked lights and not good settings. However, here a lot of limits of course.