If what you're looking for is speed, then instead of sending an ASCII encoded int, you can divide your number into two bytes, here's an example:
uint16_t number = 5703; // 0001 0110 0100 0111
uint16_t mask = B11111111; // 0000 0000 1111 1111
uint8_t first_half = number >> 8; // >>>> >>>> 0001 0110
uint8_t sencond_half = number & mask; // ____ ____ 0100 0111
Serial.write(first_half);
Serial.write(sencond_half);