ESP8266 latency via UDP - udp

Question

I create a simple setup of a master (ESP8266-12F) and a slave (ESP8266-01) and use UDP for data transmission. A message contains 4 bytes. I measured the time between the Master sends a request, and the Master gets the answer from the Slave via micros().
The Problem is there is a latency increase from 2-4 [ms] up to 25 [ms] and more.
To confirm that I remove all Serial.prints() perhaps a Serial Buffer problem, and I use as Signalindicator a digital Pin (High on Request and LOW if Answerpacket arrived) the Oscilloscope shows the same behaviour.
Next Step I thought maybe a intern Buffer increase? So I watch the Heap to each signal but there seems no problem. Perhaps a static buffer only for the WiFi???.
Next step I changed the interval-time of Masters request from 50 [ms] to 1000 [ms], but still the same picture. Also a different CPU Clockfreq. (80-160 MHz) brings no changes.
Pictures:
Red: 80 MHz Request every 1000 ms
Blue: 80 MHz Request every 50 ms
Yellow: 160 MHz Request every 1000 ms
Green: 160 MHz Request every 50 ms
Afterwards I created a new nodemcu Firmeware (Modules: adc, file, gpio, i2c, net, node, spi, tmr, uart, wifi), but same behaviour…
I thought it is a WiFi-Signal problem, so I tried some Metalshields around the ESPs, but this brings not the latency in it…
Maybe a general WiFi-Modul problem, so I tried to write commands on an SD card and measure the time… And there was the same behaviour, but not so random. Now I think there are more than one or two reasons for the UDP latency.
QUESTION:
Has anyone the same problems or have anyone experienced a behaviour similar to this?
Edit : Added code
// Master (Send Request and waits for answer)
void loop(void)
{
delay(50);
CallSensor();
do{
;
}while(!ListenClients());
}
void CallSensor(void)
{
static long counter = 0;
char requestPacket[_SIZE_PACKET_] = {0x50,0x00,0x00,0x00};
APlocal_IP[3] = SENSOR_FIELD[0][0];
if(SENSOR_FIELD[0][0] != 1)
{
TIME_UDP_START = micros();
digitalWrite(_PIN_LED_B_, HIGH);
UDP.beginPacket(APlocal_IP, UDPPort);
UDP.write(requestPacket);
UDP.endPacket();
}
}
bool ListenClients()
{
static uint16_t remport = 0;
static IPAddress remIP = {0,0,0,0};
char incomingPacket[_SIZE_PACKET_] = {0x00,0x00,0x00,0x00};
if( UDP.parsePacket() && (UDP.read(incomingPacket, _SIZE_PACKET_) > 0) )
{
remIP = UDP.remoteIP();
remport = UDP.remotePort();
if (SENSOR_FIELD[0][0] == remIP[3])
{
TIME_UDP_STOP = micros();
digitalWrite(_PIN_LED_B_, LOW);
Serial.printf(":%d\n",(TIME_UDP_STOP-TIME_UDP_START));
return true;
}
else{return false;}
}
else{return false;}
}
// Slave (sends Value on Masters Request)
void loop(void)
{
ListenHost();
}
void ListenHost()
{
static uint16_t remport = 0;
static IPAddress remIp = {0,0,0,0};
char incomingPacket[_SIZE_PACKET_] = {0x00,0x00,0x00,0x00};
if( UDP.parsePacket() && (UDP.read(incomingPacket, _SIZE_PACKET_) > 0))
{
SendAnswer();
}
}
void SendAnswer(void)
{
char requestPacket[_SIZE_PACKET_] = {0x26,0x00,0x00,0x00};
UDP.beginPacket(APlocal_IP, UDPPort);
UDP.write(requestPacket);
UDP.endPacket();
}