Why are the Map downloads so slow?

That's fine - and I did confirm in the meantime that releases (it would be 2021.10 surely?) used to occur around first half of June each year.

I have a Garmin NUVI 765T I bought in 2009 with a lifetime map update.  My internet connection is CONSISTENTLY 102 Mbps download/102 Mbps upload ON ANY OTHER WEBSITE.  When I use Garmin Express to upload my GPS it starts at 102 Mbps and quickly and steadily drops down to around 1.5 Mbps and takes well over an hour to update.  I've uninstalled the program, deleted the folders, re-installed the software...same results EVERY SINGLE TIME...as if Garmin ITSELF is the one "throttling down" the download in hopes to make a sale for an updated product.  THAT'S A FACT!!!  Just sayin'.

I agree.  Downloading updates is hit and miss, with most times being a frustrating experience.  I have a fibre connection to my ISP with 1Gb up/down and no data cap.  I have a direct Ethernet connection to my router (not WiFi).  Typically files will transfer in minutes, if not seconds (including large Microsoft program files).  The only site I have trouble with is Garmin.  As an example, my third attempt at downloading a map update this morning:  transfer started at 30MB/s and slowly dropped to about 4MB/s after roughly 30 seconds.  After 15 minutes, it was down to 1.5MB/s and only half way through the download.  After another 10 minutes, the transfer has completely stalled (but still showing 1.3MB/s).  I expect the transfer to ultimately fail.  There seems to be a common thread with people experiencing the same issue.

It is very obvious that the problem is with Garmin Servers. they are limiting downloading speed to lower their expenses. Nothing you can do.

Garmin don't throttle download speeds.

(I have a 3 hour download) The fundamental reason why Garmin's products are only 2 or 3 stars out of 5 is that they have no or very little competition. They have no reason to refine and innovate. These issues (there are many) we will put up with because we really don't have much of a choice. Garmin could fix these issues but why should they. We are going to buy there stuff anyway. I don't dislike Garmin. I'm a little annoyed them right now. Mostly just really disappointed. )  I just recently purchase a gpsmap 66 to replace my gpsmap 60 that I purchased almost 20 years ago. Aside from a few additions the 66 is basically the 60. 

New CN NA map has been released, took less than 15 minutes to download.

I also have a high end internet setup and connect to many sites for hours a day and routinely move gbytes of data for my job (in computer science...). I am now watching Garmin Express download at 1.5Mb/sec, at the very same time I did a test download from another site at over 400Mb/sec, and a GB/sec on a wired connection. Garmin may not formally throttle, but they sure as heck have a choke point somewhere in the download process: throttling, slow servers, narrow pipe, cloud files w/ low bandwidth contract, ... slow disks ... not sure what it is. Judging from how it tends to kick  in abruptly from a reasonable download rate to a suspiciously round number of 1.5MB/sec for me and others, I'd say they are running some sort of (crude, perhaps home-made) load limiter somewhere in the pipe. Maybe on servers, maybe in Garmin Express, who knows.

There's another possibility: What we think is Garmin Express showing us the download rate from garmin to our local disk, is really a combination of that, and the load from our disk to our GPS. So the burst we see at first could be the file downloading from garmin to our local disk, and the slow rate we see after that is the transfer from our local disk to our garmin device. I'm watching a painfully slow update of my Montana 700i right now, where there is no network activity and the write to the Montana is going at 1.5Mbits/sec. If you believe in numerology, that's an interesting coincidence.