@roma first I would like to correct a misconception stated in your post: Wix does not generate separate script for each element on the page. For example, the same animation implementation is used regardless of how many elements are animated on a page built with Wix. Also, while we are working on reducing the number of resources Wix sites download, since we’re using HTTP/2, CDNs, and resource hints, our delivery of resources is actually very efficient.
With regard to performance test: whether it’s ours or some third-party, these are just tools intended to pinpoint performance issues, and help site owners optimize their sites. Our insight into the internal operation of Wix sites enables us to detect exactly when they become responsive to user interactions, for example. This is also why we restrict our testing tool to Wix sites, since we cannot detect this for sites not built with Wix. Generic, third-party tools are restricted to heuristic-based, “black box” approaches that they apply to all websites. This enables them to estimate when a site becomes responsive, but not more than that.
Another difference is that we know the networks and devices that were used to access Wix sites, by collecting anonymous performance data from real user sessions. This enables us to run the performance tests using the same conditions. Other testing tools are hardwired to specific simulated environments, such as a Moto 4 using 3G.
Most importantly, we are able to provide Wix users with information that they can actually act on to improve the performance of their site. Third party tools often provide suggestions that Wix users cannot utilize.
Finally with regard to statement “considering how heavily Google weighs site loading time when its considering ranking”. While we all agree that performance is very important to the success of a page, what Google actually stated is: “will only affect pages that deliver the slowest experience to users and will only affect a small percentage of queries”, see Official Google Webmaster Central Blog: Using page speed in mobile search ranking
Specifically with regard to PageSpeed Insights, Google’s John Mueller stated: “Use these tools to find ways to improve your site for users, don’t see them as the final goal.” https://twitter.com/JohnMu/status/1092774805991051265
BTW if you check Wix site using the Google web.dev site, which like PSI uses Lighthouse, you will see that Wix sites generally get very good SEO scores. This would not be the case if Google PSI results harmed Wix page rankings.
Another BTW, have you checked the PSI score for the CNN website, or NYTimes, or even the mobile score for Amazon?