matlab serial communication with arduino is very slow

I want to send commands with matlab, to arduino uno which is connected to a 4channel relay. but after about 8 seconds of communicating, it takes so long to send commands. so arduino recieves my command late and relay will change state after about 20 seconds delay!

here is my matlab code:

sr = serial('COM4','BaudRate',9600);
fopen(sr);             % open Arduino USB port

while 1
command = input('say the command:');
if (command == 0)
    break;
end
fprintf(sr,char(command),'async');
end

fclose(sr);
a = instrfind();
fclose(a);

and here is my aruino code(void loop code). and help will be appreciated.

if(Serial.available()>0)            // if there is data to read
{
  matdata = Serial.read();          // read the data
  if (matdata==1)
   {digitalWrite(relaypin1,HIGH);
   }
  else if(matdata==2)
  {digitalWrite(relaypin1,LOW); 
  }
    
  if(matdata==3)
   {digitalWrite(relaypin2,HIGH);
   }
  else if(matdata==4)
   {digitalWrite(relaypin2,LOW); 
   }
    
  if(matdata==5)
   {digitalWrite(relaypin3,HIGH); 
   }    
  else if(matdata==6)
   {digitalWrite(relaypin3,LOW); 
   } 
     
  if(matdata==7)
   {digitalWrite(relaypin4,HIGH);
   }
  else if(matdata==8)
   {digitalWrite(relaypin4,LOW);
   }
}
else
{Serial.println('None');}