same problem here, using windows 10 64-bit, My connection is stable, I monitor it and check upload and download speed, connection is right on. .5 meg per second. by the way I do not have any problem with any other downloads. why only garmin express? I checked fire wall, no problem there. plenty of ram . it appears to me that garmin is bottlenecking bandwith, depending on how many people are downloading. this needs to be fixed and stop dodging the issue. this has wasted many days of my time. I have gone through all of the suggestions I can find. nothing has solved this problem. getting about time to try some other gps's. garmin is getting outdated rapidly. yah they give you lifetime maps and it takes a lifetime to download them.
The issue invariably when this comes up is something that isn't to do with Garmin. I've been updating maps on Garmin devices for over 10 years and never had a slow download. If you're sure it's not your ISP throttling the download call Product Support for detailed help.