If the green line above the thumbnails has completed (for video clips/not still images), then preview should be smooth.
But if continued editing is choppy, then wait for the ribbon to complete. At times it may remain blue.
When editing changes are made, such as adding effects or transition, there may be a quick redraw of the green line or ribbon.
No need to wait for the blue ribbon under the preview window to turn purple or gray prior to export.
For complex, lengthy projects use the 64-bit release.