Long delay sending data to Arduino via Serial Communication

I am trying to power a Servo motor by clicking on buttons in Unity. It works but it takes almost a second for the data to reach Arduino...

Arduino:

 #include <Servo.h>
 String received;
 Servo myservo;
 void setup() {
   // put your setup code here, to run once:
   Serial.begin(9600);
   myservo.attach(9);
 }
 
 void loop() {
   received = Serial.readString();
   Serial.println(received);
   if (received == "1")
   {
     myservo.write(180);
   }
     if (received == "2")
   {
     myservo.write(0);
   }
   
 }

Unity:

     using System.Collections;
     using System.Collections.Generic;
     using UnityEngine;
     using System.IO.Ports;
     public class Communication : MonoBehaviour {
         SerialPort sp = new SerialPort("COM6",9600);
     
         public void Send1 () {
             sp.Open ();
             sp.Write ("1");
             sp.Close ();
         }
         public void Send2() {
             sp.Open ();
             sp.Write ("2");
             sp.Close ();
         }
     
     }

The public functions are called by two buttons.

Is there a way to reduce the delay? Is my code wrong or something?

Edit: I noticed that the serial LED on arduino lights up almost instantaneously, and the servo works only after a while.... why?