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Server has a 1Gbps uplink speed, local PC has a download speed of 100Mbps. Latency is 89ms.
SFTP speed while downloading a single file from the server to the PC maxes out at 2.5MB/s, even though higher throughput can be achieved when downloading multiple files simultaneously.
After tuning the TCP window size on my server to be higher than the Bandwidth Delay Product, I was still getting low download speed from my local PC.
Windows 7 uses a default TCP window size of 16MB, but there is a bug that causes TCP Window Scaling to run in "restricted" mode due to Windows "heuristics" being enabled. The solution is to disable heuristics
After doing so, SFTP speed from the server to local PC (89ms RTT) instantly went from 2.5 MB/s to 11 MB/s