You will get higher a throughput while going at 5mph. Higher speeds indicates that the RF channel conditions such as delay and Doppler spread are changing at fast rate and its a fast fading channel. It is difficult to achieve equalization in such situation. Hence the modulation scheme can be shot down to, say QPSK(2 bits/symbol) from maybe 16QAM(4 bits/symbol) to provide a more robust transmission. This brings down the user throughput.