We appreciate your feedback and very sorry that you faced problems while working with Basel.
First of all, we would like to tell you that Google very often scores with 20-40 points mobile performance for 99% of websites built with WordPress. To get more points you need to have a really light page. For example remove all sliders, carousels and leave a few banners and products only. Then you will get a high result. But usually, our theme works very fast on mobile devices and don't have any problems with performance. We suggest you test it on a few mobile devices and you will see that there are no real performance problems. Google PageSpeed gives you recommendations on how to improve the performance of your web site, but it does not actually measure the speed (aka loading time) of your website. If your site loads fast, you can safely ignore PageSpeed’s recommendations. We suggest you read an article from the WP Rocket team - one of the most experienced companies in terms of WordPress optimization. https://docs.wp-rocket.me/article/85-google-page-speed-grade-does-not-improve
Basel has optimized HTML, combined JS scripts, minified CSS, conforms all the coding standards from WordPress (and it is checked and confirmed by Envato reviewers while uploading our theme to the market). We are always working on improving our theme's speed and listen to all our customers' reports. We release updates every 5-10 weeks so if any performance problems are reported they will be fixed asap.
So again, thank you for your feedback but we hope for your understanding and so you can fairly rate our theme. And we would be happy to do our best to help you if will have any questions related to our theme directly. If you want to return your money back, you can request a refund here https://themeforest.net/refund_requests/new
Thank you in advance.