What is the CPU/Threading usage when your exporting?
It may not work depending on your preview and export settings.
For example, from a QuickTime format (previews) to another QuickTime format (export) has often not worked. As in ProRes previews to ProRes export.
One possible fix is to use a non-QuickTime preview, like MPEG.
Be that as it may, Jim, this isn't advertised behavior. The Help documents specifically claim that using Previews will speed up exports when the output format matches the Sequence settings.
Work with preview files
When you render previews, Premiere Pro creates files on your hard disk. These preview files contain the results of any effects that Premiere Pro processed during a preview. If you preview the same work area more than once without making any changes, Premiere Pro instantly plays back the preview files instead of processing the sequence again. Premiere Pro stores the preview files in a folder you can specify. Similarly, preview files can save time when you export the final video program by using the processed effects already stored.
I don't want an inferior workaround, I want the software to work as advertised.
I'm not saying it's 'right'. Just trying to offer a possible explanation and a method to make it work until Adobe corrects the issue. (If they can. I suspect this may be Apple's fault for keeping QuickTime a 32 bit process.)
Today, I'm getting faster exports with Use Previews switched OFF. Two times faster.
I have a theory I'm going to propose: Pr creates corrupted preview files. They're rotten, I tells ya'. Rotten.