Serial port slows / freezes after a few sends.

After sending X iterations of data over a serial port via fwrite/fprintf code begins to execute very slowly, one fwrite() call per 3 to 10 seconds. (X being nearly a random number from 3 to 1000+)

I am unsure where the problem is. The serial device is receiving the data.

clear, clc

count = 0;

delete(instrfindall) 
socket = serial('COM11','BAUD',57600);
fopen(socket);

pause(3);
for i = 1:10000

count = count + 1;

disp(count)

fwrite(socket,'A','uint8')

%fprintf(socket,'%s','A')

pause (.01)

end

EDIT

Adding "stopasync(socket)" before calling "fprintf()" results in working code. Does anyone know why without "stopasync(socket)" the code fails? I currently suspect an hardware issue as the COM port is a USB port emulating a COM port.