Re: [jlreq] Optimize the images (#218)

I'm not sure whether we may have such configuration, but I think adding loading=lazy is the option. (with respec processed html file, "completed=7.04, DOMContentLoaded=5.70, load=7.05 sec" to "completed=2.17, DOMContentLoaded=1.65, load=1.79" on Firefox)

For image files, size of current English files is:
- 8.7MB in png
- 6.1MB in webp (default conversion option)
- 4.8MB in png with optimized by optipng using default option
and I don't think we can get large improvement by changing the image format. 

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