looks like you have a lot of higher latency hops closer to destination, all that adds up. Have you tried forcing the speed with the slingbox? in the client settings, put in say 1000kps and then 640x480 as the resolution, see what happens.

Sometimes the slingstream mis-interprest the latency as a problem and therefore throttles the attempted stream. By putting the number in there, it will just cram the stream down. I find it settles and buffers quite well though.

You might also, if you have it, try doing a VPN to a domestic destinaion and then making the connection. sometimes the VPN will actually speed up the traffic flow.