Why does my MBP mid retina 2012 run so slowly when using it's built in monitor?
To force the usage of the discrete graphics card in your macbook you can uncheck the automatic graphics switching checkbox in the system preferences/ energy saver panel 

As the message states, this does drain the battery faster.
However, your mac should not behave as you describe, I suggest taking it to a service desk so apple can take a look at it, or call apple to ask why it is so choppy.