[Hint] Change get_columns in admin-subpage

Hello, I was wondering why the plugin is slow, then I installed query monitor and I see that plugin generates 200 queries for DB. And its because SQL query in get_columns() method is fired for every row in the table. So I want to share a hint, extract the code from the method, create another one, call it before get_columns, store it in array is private property and then in get_column call that private property. I was able to remove all 200 queries and remain with only 1 which speeded up the page by 50%