Do you have any hidden (control-b) or turned off layers in your sequences? When you ETT suddenly FCP thinks it has to rendering SOMETHING for every instance you have a clip that’s hidden or turned off…. even though you’ve rendered it….
For over a year we always had the roughcut clips turned off in our online sequences, and wondered why it was an hour’s render for every #@$& ETT we did. Then we deleted those clips before ETT and ZOOOOOOOOOM