Here's one for you. I'm trying to transfer a large file between two houses both of which are on GF. The source house has 1G service, and the destination house has 500M service. I can't get a better transfer rate than 1.1MB/s.
To isolate network throughput from other potential bottlenecks, I would start by testing with iperf3 (https://iperf.fr). You’ll run the server at one site and client at either, and alternate, like you’ve done with other tests.
If the speed is less than you expect at either, keep testing within smaller segments (LAN at site 1, LAN at site 2, on the same switch, etc.) and map out the speed on each link to find the bottleneck(s).
If the speed _is_ what you’d expect, then the transfer could be disk I/O limited. While a single 7200 RPM drive can push 80-160 MB/s a fragmented file system, may be much slower than this.
-Aaron