fast SerialUSB for Arduino DUE

Hardware Arduino Due
3

Hi Graham! I used in PC the following python script:

import serial
import sys

ser = serial.Serial(
port = '/dev/ttyACM0',
baudrate = 115200,
parity = serial.PARITY_NONE,
stopbits = serial.STOPBITS_ONE,
bytesize = serial.EIGHTBITS
)

if ser.isOpen() == False :
ser.open()
if ser.isOpen() == False :
print "Can't open serial port"
sys.exit(1)

for i in range(100000) :
ser.write('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa')

In Arduino Due:

char buffer[48];

void setup() {
SerialUSB.begin(115200);
}

void loop() {
if (SerialUSB.available() > 0) {
SerialUSB.readBytes(buffer, 48);
}
}

Speed of read is around 1Mbit/s

How to make 10Mbit/s ? What's wrong may be in my scripts?