Apparently, when you add an item to a list using SPList.Items.Add(…), SharePoint is clever enough to first get all the items in that list and then add the new item. When you need to add a large number of items to a list, this slows down performance. What you need to do is first execute a query saying you need an empty list (RowLimit = 0) and then add your item to it. Then SharePoint will return the List-item with no listItems in it.

   1: SPQuery query = new SPQuery {RowLimit = 0};

   2: SPListItem newItem = list.GetItems(query).Add();

   3: newItem[SPBuiltInFieldId.Title] = DateTime.Now.ToString();

   4: newItem.Update();

Many thanks to René Hézser’s blog to pointing this out!