ThumbsPlus is likely decoding the WebP to bitmap for every thumbnail repaint. Fix: Generate thumbnails once, then lock them. Right-click the folder > Properties > Thumbnails > Lock thumbnails. This prevents recalculating.
Do not delete your originals. WebP is superior for web use. Keep two versions: a "master archive" of WebP (smaller storage) and a "converted archive" of JPEG for ThumbsPlus (larger but searchable). thumbsplus webp
Before manipulating ThumbsPlus to work with WebP, you must understand why this format matters. ThumbsPlus is likely decoding the WebP to bitmap