Cyclical latency changes

[MadlyFX]

Here's a fun one. Using the SocketIO library from timum-viw, I implemented these simple functions:

Server:

var interval = setInterval(test, 5000);
function test() {
  	nowTime = Date.now();
	io.emit('my_ping');
}
socket.on('my_pong', () => {
        nowTime = Date.now() - nowTime;
        console.log(nowTime);
    })

Client (ESP32):

void my_ping(const char * payload, size_t length) {
  webSocket.emit("my_pong");
}

And let it run. The result? Over the course of 30 seconds, the latency slowly creases to around 100ms, then "resets" to around 30.

31
44
58
75
94
108
28
42
58
75
92
114
22
53
57
75
90
108
25
41
56
75
92
107
25
45
64
77
97
111
25
43
62
77
94
111

What could be causing this?