A Microsoft file hosting and synchronization service.
Having the same problem, the org I work for shifted to Onedrive a few years back. We are now shifting to Google as being a no-profit, we get a lot of space for free, but trying to download the files is all over the place. I grabbed a few multi-gigabyte folders earlier today and got sort of reasonable speeds. At the moment I'm pulling down another one and getting around 5Mb/s on a 140Mb/s link.
The quality of MS's service is just rubbish, and they clearly are either deliberately reducing download speeds for large transfers to prevent people moving services, or they just have sh!tty bandwidth and have no clue what they are doing. Either way, they aren't competent to hold other people's data.