There is no reason the query should be taking longer in Tableau. Are you measuring the actual time the query took as seen in the logs? Or is it the time observed while watching Tableau update the worksheet? In the logs you should be able to find a line like "Running the query took 0.0224 sec" that tells you exactly how long the query portion took. It may be that some of the time is being spent processing the results. Above that line in the logs the actual SQL query is logged.
In addition, Tableau sometimes adds CASTs and other functions to make sure the data comes back in a way that Tableau can use. Sometimes the query optimizer in the database gets confused by these and it makes things slower. Contact support and have them look at your logs.