Yeah the problem with Bridge (and Photoshop) is that the conversion is not multi-threaded. I can do the conversion in LR 3x - 5x faster beacuse it is multi-threaded. But I have to go through the import process.
Have you explored all of the conversion options in Bridge, or are you just running a Photoshop action from Bridge? I only ask because not everyone is aware of all the bulk convert/export paths through Bridge 12 and 13. Three ways you can bulk export out of Bridge without Photoshop are:
Camera Raw. Select multiple images in Bridge, open into Camera Raw directly (not through Photoshop but using File > Open in Camera Raw), select all, use Save icon/command in Camera Raw to export to different format. The system monitor on macOS (Activity Monitor) indicates this uses multiple cores. And, if it’s Camera Raw 14.4 or later, export is also GPU-accelerated, as it is in Lightroom Classic.
Export panel. Open the Export panel, set up preset with your file conversion settings, drop multiple images onto export preset, and start export. macOS reports this uses multiple cores, but not the GPU.
Workflow workspace. Switch to the Workflow workspace, set up preset with your file conversion settings, drop multiple images onto workflow preset, and start export. I would guess this uses multiple cores too, but for some reason this method is currently failing with errors (even though it should work) so maybe this method should be avoided for now.
The point is, you should have bulk conversion options in Bridge that are multithreaded. Running a batch of images through a Photoshop action is currently the slowest and least efficient way to convert files.
However, I am forced to do the import and preview generation every time I pull in a group of images. That takes a lot of time and is unnecessary for what I'm trying to do.
That might not be completely true. Specifically, you can have Lightroom Classic skip the very time-consuming preview generation by using embedded previews (in the File Handling panel of the iImport dialog box, select Embedded & Sidecar), which simply uses the previews already embedded into the original files. Yes, it will spend time doing an ingest step, but with Embedded previews, preview generation will not be done until a Lightroom edit is applied.
But if you have no need to maintain a catalog of the converted files, one of the multithreaded bulk export paths through Bridge is probably what you really want.