slow sync speed much, much slower than my internet upload speed - is there optimal network settings?

So I've had a bit of a nightmare regarding syncing lately that is largely resolved. To cut a long story short, I ended up deleting the cloud library and I'm in the process of syncing collections again from Classic.

(My saga has been detailed here, if it's useful https://community.adobe.com/t5/lightroom-classic-discussions/lightroom-classic-will-not-sync-not-matter-what-i-try/m-p/13629080#M313492)

However, although it's working and is stable, it's very, very slow. Much slower than it used to be. I'm only syncing smart previews to the cloud, which are relatively tiny. My upload speed is a respectable 18mbps via my eero mesh to the main isp supplied modem/router (talktalk - UK isp). However, I am watching the upload speed from lightroom like a hawk and I'm getting around 3 KB/s (bear is min my 18mbps equates to a potential 2250 KB/s).

I also note that it's slow display my uploaded smart previews on both lightroom.adobe.com and the Lightroom cloud desktop app.  Other apps on my mac work just fine. Even my spare mac seems slow using the Lightroom cloud desktop app.  Again, this used to be fine before I started having all these sync issues,

Conversely, the lightroom app on my iPad pro and iPhone is super fast to download the smart previews. As is  the lightroom.adobe.com web version on my iPad.

I find this very strange. Is there some hidden network setting or process that only affects macs and not iOS devices? Or is there a port I need to open, or something I need to add to the mac's firewall to open up the full potential upload speed. Or is is simply Adobe's end?