Shadows are controlled in the Light Inspector. To improve performance, you can temporarily turn off the rendering of shadows in the Canvas.
In the Layers list or Canvas, select the light, then select the Shadows activation checkbox in the Light Inspector.
Note: To cast a shadow, the light must be offset from the object casting the shadow, and the object casting the shadow needs to be offset from the layer receiving its shadow.
Rendering shadows can significantly impact playback performance. You can temporarily disable shadows to improve playback speed while working on other aspects of your project.
Click the Render pop-up menu above the Canvas, then choose Shadows (or press Control-Option-S).