Prophoto 6 and serving WebP images

[bucz]

Ok, so after a few years, I decided to update my website and sort out some performance issues (score around 50 on mobile). I'm not skilled in this area and I'm learning as I go along. My website ( https://stradphoto.com/ ) is on Prophoto 6 which they abandoned and want a monthly fee now to get support and new features which is not the cheapest.

I have been playing with the Smush plugin for WordPress and I have optimised all the photos in the Media Library. Unfortunately, there is an issue with serving the WebP format. I have been told by the plugin support

This is more likely to be some plugin or theme conflicting on your site. I suggest to perform full plugin/theme conflict test in order to find the plugin or theme causing this.

I have switched off all the plugins and the problem on PageSpeed Insights mostly for mobile stays the same - Serve images in next-gen formats - which means it could be the theme conflict. I don't know how to do this yet. They say to do this in dev/staging site.

Please make sure you have recent backups and perform this operation on your dev/staging site if possible.

To learn more about a plugin conflict test, please see this guide below:
https://wpmudev.com/docs/getting-started/getting-support/#conflict-test

Check out this handy flowchart of the troubleshooting process detailed above.
https://wpmudev.com/wp-content/uploads/2015/09/Support-Process-Support-Process.gif

Has anyone had this issue and managed to sort it out?