Power Query Refreshing

Hey everyone! I'm new to the Power BI Community and I'm running into an unexpected issue. I'm using Power BI Report Server and connected to and importing a table from Oracle DB. The problem is, everytime I edit a column or make any tranformation within the data(e.g Replace values), it refreshes the data from the data source, reading the complete database each time and reapplying the filters. As you can imagine, with millions of rows this is causing immense wastage of time and is a serious set back for us. Is this supposed to happen? Or could I be doing something wrong?