When selecting boxes during import page loads are slow. Having the option for All/None does not help if you only want a few items in one topic. Example: You have 20 topics. All topics contains 10 or more items. You only want to import content from 3 topics. During the import you choose 'None' and then choose to click the main topic box for the 3 topics you wish to import. This does not automatically check all items in that topic and you must check each item individually.
In the same scenario, different steps. You Choose 'All' during import and then choose to uncheck all topics except the 3 you wish to import. When you check a box you must wait for page to loads before you can check another main topic box.