Time taken to drop/add index is lesser than adding a column. Why is that?
Let me say in a simple way. When you create a new  table/index and then you are inserting 10 rows so, index value will be 1 to 10. Now you are deleting 5 and 6 the row in the table so ,holes are formed. Next time while inserting rows from 11 to 20 then index 5 and 6th rows need to be refilled that's why it's taking time. If you drop a column and recreate index then value is auto increment that's why it's faster and there is no hole.