why speed decrease? How can I keep the first transfer time for ever?
The first few time all print has to do is stick the characters in the buffer. Going out from the buffer to the Serial line is interrupt driven. So it just copies chars into the buffer and returns. Super quick.
Later once the buffer is full, print has to wait while chars are sent out to make room for the new stuff. That takes some time. If you increase the baud rate it goes faster.