Also any layers or clips you have Hidden or turned off in your timeline will have to be “rendered” when you ETT. Even though the clip has no bearing on your output, FCP spends time “rendering” it.
I love to keep a strand of the rough cut on a high video layer, turned off, for reference. little did I know that was causing enormous renders at ETT. I thought FCP was doing a a mixdown or something. Now ETT is nearly instant.