You may need to see if "servername" maps to some other IP address than 192.168.0.10, and then figure out what route through your network your packets would take if you connected to that other IP address.
For example, your address of 192.168.0.10 suggests to me that you're running a NAT gateway on your network. Let's say "servername" is an unqualified DNS hostname (fully qualified, it might be servername.domainname.com). If servername.domainname.com points to the public IP address of your NAT box, then when you connect to \servername, your traffic from your client to that server is going out through your NAT box towards its public port (outbound NAT), then just before your NAT box sends it out its public port, it realizes that that's its own IP address, and then does inbound NAT (a.k.a. port mapping, port forwarding) to send that traffic to your server. This process of outbound and inbound NAT on the same traffic flow is called "hairpin NAT". I wouldn't usually expect hairpin NAT to destroy your throughput quite that bad (30MB/s to 400KB/s), but if it's a cheap home gateway I suppose it could happen. It could be that introducing a hairpin NAT between your client and server is screwing up TCP congestion avoidance and retransmit algorithms in some way.