I had this same problem on Studio One 4, 5 and 5.1. I finally solved the problem by changing my monitor's G-SYNC settings. I have my monitor set to 144hz refresh rate and turned it down to 60hz, then noticed that the "mouse lag" I was seeing in Studio One was how everything looked at 60hz; this led me to wonder if Studio One was somehow rendering at 60 frames per second when my monitor was at 144hz.
I opened Nvidia Control Panel and navigated to Display > Set up G-SYNC, then changed "Enable for windowed and full screen mode" to only "Enable for full screen mode." This fixed my problem immediately! It seemed as if something about G-SYNC running on Studio One is causing it to have some video tearing or to render at 60fps for some reason.
I don't know if you are running on a G-SYNC monitor, but try this if you are. If not, maybe try playing with some other settings related to refresh rate (FreeSync?) on your machine.