Speed limitations for TCP connections in different Availability Zones.

TCP performance can be greatly influenced by latency. There's not much you can do about the latency between London and Singapore - the AWS network will route via the quickest path.

Depending on how much you can control the protocol you're using, you might have better results with UDP because it is affected less by latency because there are no mandatory acknowledgement packets. If not, you might consider transferring data using multiple parallel TCP streams.