I gotta think the owners know HOW to fix this and just are not, for some reasons.
There is no reason to believe they know how to fix a problem that only you seem to have. They have to be able to experience it or at least get some diagnostics in order to address it, if it is at their end.
So far we have heard from several users who are using Safari without your issues, and none who are seeing them. This definitely implies the problem is your end, but if you are not seeing slowdowns on other sites I don’t know what the explanation could be.
I have a new Mac, a newish iPad, and a 4-year-old iPhone without any issues. However, I use alternative browsers (DuckDuckGo and Firefox) so I cannot compare notes on Safari.