I wouldn’t use WebP images. They’re not supported at all by Safari (Apple devices) older versions of Firefox, or any versions of IE. Also, there’s not much anyone can do with them. They aren’t easily shareable, and they can’t be easily opened in many graphics applications. To maintain compatibility with older browser would necessitate adding a lot of additional code to your site, which would only slow it down further.
Page load speed is determined by numerous factors, not just the size of your images. If your page if very graphic-heavy, you may want to lazy-load the images, so the area above the fold renders first.
Without seeing your actual code, it’s hard to say what factors are impacting your page load speed.