One thing to consider is that typically the transmitter in a home satellite setup will be low enough power that sufficient forward error correction is necessary. It's also typical that this kind of forward error correction requires a best of 4 redundancy. So although what you see is 3Mb upload speed, the actual data stream is approximately four times that. Bottom line: The low rate you're seeing is simply an engineering tradeoff to achieve an acceptible level of transmission reliability that's not necessarily required in the receive direction.
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