Hi Roger,
I think Mac OS is (was) based on Berkley (BSD) which is similar to Linux, (both were intended for Intel processors), but not quite the "big" Unixes of old.
The DC uses Safari on her (last year's model) iPad, and I find it slow and clunky. That said, there are a number of sites out there that use an extreme amount of cookies, but worse, include "iframes" and similar which present stuff from other web sites entirely. Examples include many news sites especially (those "sponsored" sections, actual adverts, and so on). All those really hit page load times badly, and depending on the page rendering going on, you may be hanging about waiting for one obscure page element to turn up, when the rest of the page arrived a while back - if you see what I mean. It's the modern curse of advertising-driven "free" content.
I'd suggest an experiment, before digging into Linux (much as I love it myself): try using Firefox (or Google Chrome) instead of Safari, to see if it makes a difference. Both are pretty quick nowadays, and a useful comparison.
You may find (and I suspect), it's modern sub-optimal web page design that's the big nuisance.
I'd also check the speed of connection to the specific sites you are finding to be slow: open a terminal window and use ping to check the round trip time (unlike a PC, which runs for a bit and times out, I think the ping of MacOS will repeat forever until you ctrl-C to stop it - same on most Unixes and linux). This won't tell you a huge amount, but if you have a fast site and a problem one, you might see an obvious difference. This is arguably more realistic than running Ookla speed tests or similar, as you are checking a server you actually use.
Note that streaming services, such as Netflix or Amazon Prime Video should do a lot of buffering, and can reduce their bandwidth requirements automatically if connections are poor. Things have to be pretty rough before the picture starts to stutter, etc. Unless you insist on 4k pic quality, the video streams they use should be well within the capabilities of most ADSL connections, and in any case the server loading will be the biggest factor at the moment. Point being: don't regard them as a definitive guide either way.
HTH, E.