For the last ten years, as described in Really Slow Upload...only Dropbox | The Dropbox Community and https://www.reddit.com/r/dropbox/comments/6n74lu/dropbox_slow_on_fios_gigabit_connection/ Dropbox has been incredibly slow on Fios - as in, less than 5 Mbps.
If I simply set a SOCKS5 proxy in the Dropbox preferences so my connection goes through some other host instead of directly to Dropbox - then I get 100+ Mbps.
This is not about the bandwidth limit setting. If I take my computer to a neighbor's house that is on Xfinity, it works fine. If I stay at home, but route the connection using SOCKS5 through someone else's non-Fios host first, it works fine. The problem is only if the connection is direct via Fios.
This problem has been going on for more than a decade, for a majority of people on Fios, at least in the northeast US.
How can this get fixed? There is clearly a peering issue somewhere.