There’s a lot of image optimization opportunities in your website, but that can only be done at the origin level. For instance, your website has many PNG images that should have been originally saved instead as JPEG or GIF. I’ll give you two examples:
This image has photographic content. PNG, like GIF, should not be used in images with photographic content. The format for photographic images is JPEG, which is a compression format created specifically for photography. Then of course you can use lossy mode and enable webp.
Just by opening the image above and saving it as JPG, without any change other than set the quality level at 65%, the image size shrunk from 146 KB to 16.3 KB, ~ 1/9 of the original size without any loss in quality.
The PNG format locks in the image in a lossless mode that won’t be touched by Polish, which will only strip off any EXIF data attached to the image.
Also this image:
How many colors can you see there? I only see two, though there might be a few shades of grey in it, not sure. But the image carries 16.4 million colors (24 bits per pixel). By converting it to GIF with 8 bits per pixel, the size was reduced from 9.1 KB to 4.9 KB. Then I reopened as GIF and saved it again as PNG and it squeezed even more, to 4.64 KB. And I’m sure both this image and the JPG image above would be reduced even further if I had used a professional image manipulation software, such as Photoshop or the open source GIMP.
The choice of format and some image quality settings, like the depth of colors, must be done as part of your image process workflow at the origin level. I see Polish as the final touch in this process, squeezing whatever it can without making decisions about file format.