Slow home internet after installing new MAC mini M4

Hi All, I'm a newbie to the MAC world, having just taken the plunge to get a MAC Mini M4 after my old Windows PC became unsupported. I have installed it but now my whole home internet connectivity is desperately slow. I have a few smart TV's and Sonos connected to a BT home hub via a switch. I didn't install it as I am not that computer capable (I can press an on switch and use a keyboard). However, prior to the arrival of the MAC, my network had been performing fine for 3 years. I did set up services such as icloud/icloud drive on the MAC and installed Backblaze, thinking sync/backup might be a good idea. I did reset the BT router once and the speed shot back up for a few minutes then took a dive. BT have told me that not their problem, line in is good but that the switch on the back end is causing all the problem "by dividing up the bandwidth into small pieces that will be individually slow." My response to this was that everything has been fine for the previous 3 years, seemingly until the arrival of the MAC (unless this is just a coincidental red herring?). Is it simply a case of turning off all cloud/backup services or has the MAC done anything to any of my settings behind the scenes that I wouldn't be aware of? Thank you to anyone that can provide a layman's answer to an archetypal layman.

[Re-Titled by Moderator]

Original Title: New to MAC. New MAC Mini M4 but now my home internet connection is desperately slow