A new study I found is https://www.doubleclickbygoogle.com/articles/mobile-speed-ma... "53% of mobile site visits are abandoned if pages take longer than 3 seconds to load"

Maybe you can create a video showing how their pages take longer to load.

I dealt with a customer once who had broken Javascript (redirect) on the homepage which rendered the page just empty. Only Firefox browser affected. No amount of emails, screenshots, sending patches or CEO-to-CEO in-person meetings helped. Their logs showed little Firefox users so it wasn't a priority. Took over a year to fix. Some customers are resistant to feedback.