Convert GrayScale to RGB

Another possibility is using the bulk export features in Adobe Bridge or Lightroom Classic. The Export panel in Bridge is shown below, with an Export Preset set up to convert all selected files to Adobe RGB TIFF with ZIP compression at 8 bits per channel. Once set up, you simply drop a large number of files onto the export preset and click Start Export.

The advantage of bulk exporting with Bridge, Lightroom Classic, or Camera Raw is that they are multithreaded and (some are) GPU accelerated, so the more CPU cores you have, the more images get processed in parallel. A Photoshop action opens and processes each image one at a time, and that is going to be a lot slower, a form of the various types of Photoshop overhead Stephen mentioned.

However, I am not sure how reliable Bridge Export is for very large batches like 20,000 images. You’d have to test it.

I agree that if the Adobe options don’t work out, the command-line options Stephen mentioned may be best. For example, ImageMagick runs back-end bulk image processing for a large number of applications and commercial websites, so it’s definitely industrial strength, and you’ve probably already been using ImageMagick here and there without realizing it.

The standard approach is to use an action and batch, or an image processing script.

You will need free hard drive space, plus extra for scratch memory. The saved images now contain x3 the channel data.

Photoshop has many overheads which often conflict with batch processing large volumes of images. Photoshop can do this, however, it will likely take longer than other software and you may have to run in smaller batches as it may not be realistic to process this many files in one go.

For 20K images, I would suggest a CLI-driven program such as ImageMagick or GraphicsMagick or  XnConvert or nConvert etc.

@Erik28443335yqob 

Are you using Photoshop Fix where you posted and which has been at end of support for two years or Photoshop?

Photoshop Fix can't do this, but Photoshop can. Let us know which application you are using and we can move your post.

Jane