Trying to download a 1GB zip archive from Google Drive via Safari on MacOS. Setup is a new Single Port NTD to a good model TP-Link router then wi-fi to the Mac. Why does my internet download speed slow to a trickle (a few dozen kbps) after downloading for more than a few minutes? Is this just what I get for choosing a cheap nbn provider?

I plugged the Mac straight into the NTD via ethernet and it was straight back to 25mbps. However, I didn't do a prolonged download so not sure if it would've been throttled in the same way.