PaulS:
Would decreasing the size of the buffer to the size of frame decrease the time needed to send the package?
No. Writing 8 letters on a sheet of paper takes the same amount of time, regardless of the size of the paper, doesn't it? Writing 8 bytes into the buffer, and then waiting for the buffer to empty takes the same amount of time if the buffer is 10 characters as it does if the buffer is 64 characters.
Since I'm sending always the same amount of bits (1 frame) to the buffer and than flush it immediately after calling Serial.write(), than filling and emptying of the buffer takes the same amount of time. Why does Serial.write() + Serial.flush() take more time than calling Serial.write() only? Shouldn't it be contrary? Sending only 1 frame and flushing it should take less time than filling the whole buffer with frames and than emptying it, right?