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Sorry for the late reply!
If you look at smartsection/class/item.php you will find that the get() method is overriden and calls assignOtherProperties();
The assignOtherProperties() will get a category and a permission handler and perform extra logic.
The permission handler will get permissions for that item, is not a problem.
The categories handler will get all smartsection categories as objects and if you look into it, the getObjects() is also overridden to get corresponding permissions. Since it use static variables this will happen only once in a request but it will still be a problem for large database sets.
Bottom line, while you think you are asking for just one object, you are asking for hundreds of them.