scp on 12.1p1-RELEASE is painfully slow:
2% 80KB 4.5KB/s 11:25 ETA
Network is 10 GBit/s (both hosts are on ESXi). net.inet.tcp.cc.algorithm is newreno (default). No firewall, scp client is on the same network.
Please supply more details on your network configuration beginning with used network cards and drivers.
Both hosts use IPv4 (on the same subnet) and vmx network card driver.
Does it help if you disable various offloads for network interfaces such ax tso4, rxcsum, txcsum etc.? Use: "ifconfig vmx0 -tso4" and so on for both sides.
Use "ifconfig vmx0" to verify that offload features got disabled, repeat test and report back.