1. // libraries
   import processing.serial.*;
   // serial connection
   Serial port;
   // toggle light
   byte toggleLight = 0;
   // time
   int sendAt;
   
   void setup() {
     // size
     size(200, 200);
     // print all available serial ports
     println(Serial.list());
     // init connection to arduino
     port = new Serial(this, Serial.list()[0], 57600);
   }
   
   void draw() {
     // clear background (check toggle light)
     if (toggleLight == 0) {
       background(0);
     }
     else {
       background(255);
     }
   }
   
   void mouseReleased() {
     if (toggleLight == 0) {
       toggleLight = 1;
     }
     else {
       toggleLight = 0;
     }
     sendAt = millis();
     // write message to arduino
     port.write(toggleLight);
   }
   
   // serial event
   void serialEvent(Serial p){
     // get message till line break (ASCII > 13)
     String message = port.readStringUntil(13);
     // just if there is data
     if (message != null) {
       println("response time: "+(millis()-sendAt));
     }
   }

So if you have large delay with this code, then it should be something with your hardware-setup. Otherwise, please post your arduino- and processing-code, so we can have a look and maybe fix it.