This workflow has been broken for a long time and I would really love to fix this one. I know this used to be on uservoice but I can't find the old bug.
1. Set sequence preview file to "Apple ProRes 422HQ" or whatever your favorite is.
2. Render Sequence to full green.
3. Go to Export, Select an H.264 preset, check "use previews"
4. Watch as PrPro doesn't acess a single preview on disk and instead renders the whole timeline from scratch.
From my current example I have a .png sequence from blender than I've rendered to green in the timeline.
Export->H.264, use previews checked...
Look at windows resource monitor, why is premiere chewing up PNGs when it should just be referencing the preview files I JUST RENDERED.
.aecache files being acessed for no reason.
.png sequence being read. WHY.