Currently you can't add values to a multi-reference via the tl:TLObject interface - you can only get the old list, copy it, extend it and set it again. For long lists this is an incredibly expensive operation. When importing thousands of objects, this leads to the import not finishing because of this quadratic complexity.
When inserting in a multi-reference, use the com.top_logic.element.meta.AttributeOperations.addAttributeValue(TLObject, TLStructuredTypePart, Object) utility.
Refactoring, no test.