Why does my MBP mid retina 2012 run so slowly when using it's built in monitor? I'm having severe performance troubles when I'm using the laptop's built in screen as opposed to an external display. 

Whenever I stop using the external display, many of the OSX animations become slow and choppy.

I suspect what's happening is that my external display always uses the discrete graphics card for acceleration. I also sometimes work quite close to the limit of my physical RAM. This probably makes it hard for my integrated graphics card to work efficiently.

Any suggestions on how to force this machine to use the discrete graphics card EVEN when not using an external monitor?

Specs below:


  Model Name: MacBook Pro
  Model Identifier:   MacBookPro10,1
  Processor Name:   Intel Core i7
  Processor Speed:    2,3 GHz
  Number of
  Processors:   1
  Total Number of Cores:  4
   L2 Cache (per Core):   256 KB
  L3 Cache: 6 MB
  Memory: 8 GB
  
  NVIDIA GeForce GT 650M
  
  Intel HD Graphics 4000
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.