Thank you for implementing it!  I'm not in a big hurry about when to do the conversion, but I'm glad that it's in your queue.

Also, another thing that I noticed when I ran the plugin on my (much smaller) test site was that it replaces the filename field completely when converting from the original format to WebP, rather than just changing the extension from the original to .webp.  I tend to keep my image filenames a certain way, and so I'd prefer to have the option of retaining them if possible (vs. going in behind the conversion and changing them back).

So I updated the plugin and took it for a spin with my "1998" album, which contains four photos.  Unfortunately, it did not work as expected.  Here's the result that I got:

https://imgur.com/a/results-from-trying … es-kVk6SFQ

I was surprised to see that it still registered the full 22,416 files in its count after I set the album.  After I started it, it behaved in the same way as before, running for a few minutes and then delivering a failure result with the files' being unchanged.

Thanks for testing it, and sorry it tripped you up — I found the cause.

After updating, your browser was still running the old cached admin.js, so the new "Album" dropdown was there but did nothing: the count stayed at 22,416 and it tried the whole gallery again, then timed out. Exactly the "same as before" you saw.
 
I just released v1.4.4, which fixes this (the script is now cache-busted) and also hardens the bulk run against host timeouts. To get going:

1. Update to v1.4.4.
2. On the plugin page, hard-refresh: Ctrl+Shift+R (or Cmd+Shift+R on Mac).
3. Pick the "1998" album — the count should drop to 4 and convert in seconds.

If the count still doesn't change after the refresh, paste the text after "Conversion failed." and I'll take another look.

Daniel