Proper Ping

They only send the latency value to clients once every 30 seconds:

public void tick() {
   if (++this.sendAllPlayerInfoIn > 600) {
      this.broadcastAll(new ClientboundPlayerInfoPacket(ClientboundPlayerInfoPacket.Action.UPDATE_LATENCY, this.players));
      this.sendAllPlayerInfoIn = 0;
   }
}

And this is how they "calculate" it:

int i = (int)(Util.getMillis() - this.keepAliveTime);
this.player.latency = (this.player.latency * 3 + i) / 4;

["Ping HUD"]
	#Enable ping HUD
	pingHudEnabled = true
	#Additionally show 5 last ping values
	showPingQueue = false
	#Draw text with a shadow
	drawTextWithShadow = true
	#Draw ping either on the left side (true), or on the right (false)
	leftOrRight = true
	#How many scaled GUI pixels to start drawing from the edge of the screen
	#Range: > 0
	horizontalPadding = 4
	#On which line counting from the top to start drawing from
	#Range: > 0
	lineFromTop = 1
	#How many scaled GUI pixels to start drawing from the top of the screen
	#Range: > 0
	verticalPadding = 4

[Other]
	#Show numbers in milliseconds in player list
	playerListNumbers = true