This article describes how a 60 Hz monitor refresh rate can interfere with playback in Final Cut Pro.

While playing clips in Final Cut Pro, you may notice dropped frames if the monitor is set to 60 Hz. To avoid this, be sure to set your monitor to a refresh rate above 60 Hz using either the Monitors control panel or the Monitor Resolution control strip module.

When setting a new refresh rate, 75 Hz is acceptable, but 85 Hz provides the best playback performance.