Same here. I have 200MB download and I NEVER have this much trouble downloading anything. Garmin, you need to fix this. It's obviously on your end. All these people just can't have bad systems. September 2019 and this has been on the forums for 5 years with no plausible explanation. What gives Garmin?
Do not buy this sat nav. Completely useless. By the way negative posts are being moderated and removed so use other social media. I am so angered by this rubbish I have taken up a campaign online to prevent people from buying Garmin products.
This site seems littered with your replies saying that there isn't a problem with download speeds, yet the number of threads rather suggests that there is.
I'm also in the UK on an un-throttled Vodafone connection getting 36Mbs with a fully copper/fibre connection (no WiFi). Garmin maps are coming down at 2.8Mb/s. Like other users, I don't have any issues with other data downloads - just Garmin updates.
I don't see any evidence that my ISP is throttling particular traffic. I would be interested if you had evidence to the contrary.
FYI - I have been in performance testing for c 20 years.