As I'm unable to reproduce this issue on my Mac running El Capitan, here are some suggestions to resolve this issue...
1. Go into your System Preferences and click on "Display." Set the resolution to "Default for display" rather than scaled if you have changed this option.
2. Try creating a new song and see if the problem still persists. I say this because you may be getting a lag in your graphics if you are using a lot of your system resources.
3. Try resetting the NVRAM on your system. Here is an article from Apple that explains how this is done.
https://support.apple.com/en-us/HT204063
4. In the event that you may have a corrupt user profile, try creating a new user account, log into that account and see if the problem persists.
5. Try going into Studio One>Preferences>Advanced>Services and disable the TUIO service.
6. If you are using a USB to HDMI adapter rather than a graphics card for an external monitor, don't do so. Disconnect any 3rd-party monitors if you are using them and just use the display on the MacBook.
7. Take your computer to an Apple Store and have them check out the graphics card and hardware.
