PHP Modules Required to Sync

Hi. I recently synced image files to a bucket using wp-stateless. It didn’t sync any thumbnails, only the original full-size image file. I then installed the gd module for php. Ran the sync again and this time it took about 5 times longer PER FILE and the thumbnails ended up in the bucket.

I’d like to understand exactly what is going on here. If wp-stateless is only moving or copying files from the local disk to a bucket, why would it need gd at all, and why would it perform some additional processing for each file when gd is available? Is it regenerating thumbnails as it moves/copies to the bucket? If so, why? Can’t it just copy the files as they are without having to regenerate thumbnails (the thumbnails have already been generated so why not use them?)

I’m not complaining, wp-stateless is awesome. But it would be useful for my load-balanced setup to know what is going on and why the gd module is needed.

Thanks for your help. Russ.