ESP8266 - UDP send interruptions

I'm trying to transfer data from ESP8266(NodeMCU ESP12E) to my laptop. I'm coding ESP using Arduino IDE. I am able to receive correct packets (each containing 400 bytes) on a visual basic console program. However, to see whether the transmission is continuous or not, I have toggled an output pin whenever UDP.endpacket() returns 1. I observed waveforms on DSO. I found that UDP.endpacket() does NOT return 1 many times and the there is no transmission from ESP for a significant amount of time (milliseconds). Refer attached waveforms.

Why does UDP.endpacket() return 0 for such long time? Why is UDP transmission getting interrupted? How can I solve this? Please feel free to ask for more details if necessary.

Code :

#include <ESP8266WiFi.h>
#include <WiFiUdp.h>

const char* ssid = "Connectify-me33";
const char* password = "donfour276"; 

WiFiUDP Udp;
unsigned int localUdpPort = 65000; 

unsigned int dest_port = 11000;

unsigned char clientBuf[400];
int clientBuf_len=0;

#define test_pin 5    

int return_udp_fcn = 0;


void setup()
{

  WiFi.begin(ssid, password);
  delay(5000);

  Udp.begin(localUdpPort);


  pinMode(test_pin, OUTPUT);
  digitalWrite(test_pin, HIGH); 

  pinMode(LED_BUILTIN, OUTPUT);     
  digitalWrite(LED_BUILTIN, LOW); 
  delay(1000);
  digitalWrite(LED_BUILTIN, HIGH); 
  delay(1000);
  digitalWrite(LED_BUILTIN, LOW); 
  delay(1000);
  digitalWrite(LED_BUILTIN, HIGH); 
  delay(1000);


}


void loop()
{
        digitalWrite(test_pin, HIGH); 

        //filling up buffer
        for(clientBuf_len = 0; clientBuf_len<=399; clientBuf_len++)
        {
            clientBuf[clientBuf_len] = 0x7E;
        }

        //UDP transmit
        Udp.beginPacket("172.22.0.1", dest_port); 
        Udp.write((const uint8_t *)clientBuf, 400);
        return_udp_fcn = 0;
        return_udp_fcn = Udp.endPacket();
        if(return_udp_fcn){
          digitalWrite(test_pin, LOW); 
        }
        delayMicroseconds(100);

}