Improving the video frame rate with an external screen

I bought Djay Pro yesterday for a VJ set I’m doing tomorrow. I’ve bought some music videos on iTunes to play, but the frame rate is terrible (~15fps at a guess) on my 2014 Retina Macbook Pro when using an external screen.

I’ve tried disabling graphics switching so I always use the discrete graphics card, and I’ve turned down the resolution on the second screen, but I’m still disappointed with the frame rate.

Any tips for making it run more smoothly?