Managing Your Work Environment > Setting Preferences > Timeline Preferences >
Soft Effects

Set the Preview FX preference when working with clips on the timeline.

Cache Preview FX button
  Enable to cache frames already rendered on the timeline when frame-stepping through effects.
Soft Effects Rendering option box
  Set to either Layer Based Render to render one layer at a time, beginning with the entire bottom layer, or Frame Based Render to render the layers in each frame before rendering the next frame (this option may slow down rendering time).