I think I've fault found a cause and may have fixed this problem - so glad, as I think Safari is way better than any other browser - and I've tried them all.
I divided my WiFi channels between 2.4Ghz and 5Hhz on the router, as I had some of my "stuff" which could connect to the 5G WiFi and some could ONLY to the 2.4Ghz - mainly an old iPhone and a Google ChromeCast - they wouldn't even "find" the available 5G network. It seemed logical to me but then a tech at my broadband provider found this;
> I can see connection is stable with good speed in line. it seems there is issue with router signal frequency,
> Please click on Wifi>> Settings>> change band to 20/40 mhz (For 2.4 GHz)
Effectively, doing this allows the router to "serve" both 2.4 and 5 Ghz devices when they connect to that one WiFi network. Switch off the (second channel) 5Ghz network and you'll find your old devices still connecting at their limit and the newer ones at the higher speed. (You may have to provide a password for them to do this)
Having just the one network (SSID) capable of doing that meant that all the "bandwidth" that the router possesses was dedicated to just that one channel. Funny thing though, I (THINK) remembering in the beginning, connecting my MAC to Ethernet and experiencing the degrading video quality?? So, there is still something unexplained but, the change has removed this problem from the browser (whichever one you use).
Internet Speed - <https://fast.com/#> and Ookla SpeedTest (50Mbps down 13up) are the definitive way(s) to measure your internet performance - BUT, they just gave impressive performance numbers and didn't indicate the network maybe at fault. However, Apples network performance tool gave a hint - that motivated calling the broadband provider;
In macOS open terminal app and type networkQuality -v
Result;
MacBook-Air ~ % networkQuality -v
==== SUMMARY ====
Upload capacity: 12.922 Mbps
Download capacity: 15.378 Mbps
Upload flows: 12
Download flows: 12
Responsiveness: Low (171 RPM)
Base RTT: 27
Start: 05/01/2022, 18:10:56
End: 05/01/2022, 18:11:07
OS Version: Version 12.1 (Build 21C52)
Try this fix, it worked for me.