When depth of field is employed, playback performance may be significantly impacted. To alleviate this issue, you can disable the effects of depth of field settings while working on other aspects of your project.
Click the Render pop-up menu in the upper-right corner of the Canvas, then choose Depth of Field (or press Control-Option-D).
When a check mark is visible next to the menu item, the effects are rendered. When no check mark is visible, all objects remain in focus.
Select a camera in the Layers list or Timeline, then, in the Camera Inspector, set the DOF Blur Amount value (in the Depth of Field section) to 0.