Serial write time

hi,
i trying reduce time taken serial.write(...) function. theoretically, time taken function should 9 / baudrate send 1 byte. used following code verify :

code: [select]


void setup(){
  serial.begin(115200);
  serial2.begin(300);
}

unsigned long t;
int i=0;

void loop(){
  if(i==0){
    t=micros(); // line a
    serial2.write(byte(5));
    serial2.flush(); // line b
    t=micros()-t; // line c
    delay(50);
    serial.println(t);
    i++;
  }
  else{
    if(i==1){
      t=micros();
      i++;
    }
    else{
      if(micros()-t > 1000000){
        i=0;
      }
    }
  }
}


this code sends byte every 1 sec , prints time taken between line , c. problem when comment line b, time taken instruction "serial2.write(byte(5));" 20µs, every baudrate... when uncomment line b, time taken approximately 20 + 9/baudrate µs every beaudrate.
in opinion, because first line copying byte write buffer, , second waiting sent.

1) assumption right ?

2) why take long copy byte (20µs !) ? want make quicker because constant time problem send data @ high speed (i send data possible , using baudrate of 460800 loose half of time copying in buffer)

any appreciated  :~

quote

theoretically, time taken function should 9 / baudrate send 1 byte.

start + stop + 8 data = 10.

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