I've added WebP (developers.google.com/speed/webp) support for the displayed images in the Grid format. Moodle itself does not support this new image format (at the time of typing) as it is not a known mime type, so you won't be able to upload WebP images but the format will do the conversion for you. The mime type 'image/webp' is served and you will need a supporting browser (caniuse.com/#feat=webp).
WebP is a new image format for the web that reduces the size of images, therefore page load times should be faster.
To use, change the 'defaultdisplayedimagefiletype' setting to 'WebP' as an administrator as per the screen shots attached. If things don't work out, then just change the 'defaultdisplayedimagefiletype' setting back to 'Original' and things 'should' be fine again.
As this is a 'pre-release' you can initially get the code from github.com/gjb2048/moodle-format_grid/releases/tag/V184.108.40.206. I strongly suggest that you only use a test server in the first instance. If all goes well, then I'll release formally in the plugins database.