Hello Andy,
sorry for my late answer, I have been moved to another project. However, I still have to solve this problem.
I tried to change my cables, also tried to program them via UART directly in half duplex, but I had the same latency of 20 ms. However, if I move my communication in Full duplex, the programming time is much more faster.
The reason behind it is that there are more than one packet sent in full duplex, while in half duplex only one byte per time is sent. The distance between the packet is still 20 ms.
Is it possible to reduce this latency of 20 ms? Perhaps in windows? I have Windows 7 64 bit.