When you export from one store and import to another, the import file includes items with their IDs. If you would import that same file into the existing store (like when you update some items), that ID allows to identify existing items very quickly, so the import speed to the same store is very good.
But when you copy your data to another store, those IDs don’t exist in the other store, so that causes the app to make an additional request to API to find that out. You can improve the speed by checking the “Ignore ID” box and that will tell the app not to try to use those IDs.
It will improve your import speed by at least 20%.