ArshadStudio Bulk Image Converter to WebP adds a dashboard screen that walks you through three steps:
The plugin performs genuine pixel-level re-encoding using the Imagick extension (preferred) or the GD extension — it never simply renames a file extension. Every output file is verified against the WebP binary signature before it is accepted, so you can trust that the result is a real WebP image.
Conversion runs one image per request over AJAX, which avoids PHP execution-time limits on large libraries.
When “delete originals” is enabled, the Media Library attachment is repointed to the new WebP file and its metadata is regenerated, keeping the library consistent.