Slide Background Fill issue(Preview and Publish unaffected)

This is a common, though frustrating, limitation in the Storyline editor, likely related to the rendering overhead of multiple "Slide Background Fill" shapes within the WPF-based authoring environment, as you've correctly inferred. Since the performance issue doesn't affect Preview or Publish, it confirms the slowdown is isolated to the editor's redraw process, not the final output quality. A primary workaround is to group your background-fill shapes and then save the group as a single PNG image with transparency, replacing the original shapes to drastically reduce the editor's redraw load while preserving the desired visual effects like shadows and transparency. This "baking" process should significantly improve authoring performance.