The list of issues with the Apple/Google relationship has a lot of unsubstantiated claims, such as sharing full web browsing history. Does anyone have a reference to where that claim comes from?
Their claim that Safari has gotten slow is that other browsers have gotten better, not that there has been any negative performance changes. I would suspect that HTML rendering as well as Javascript execution have significant diminishing returns on performance improvement investments.
Their first reference it there being issues with Web Extensions was to some person's random GitHub issue tracker, which isn't being updated when issues are fixed and doesn't necessarily provide a FB number to show it was raised to Apple. This IMHO is somewhat Apple's fault as they haven't contributed a Web Extension API component to WebKit; it is a Safari feature and needs to be raised as Feedback (which is then not externally referencable, and may not be updated as internal duplicates of the issue are closed).
The 4th thing in the article title/headline "Dear Safari, 4 Things I Hate About You"... seems to be missing?