> - l: Probably some length?

I don't understand why the author didn't just look this up in the source code. Lichess is open source and we can see exactly what this field is here, it's the average lag:

https://github.com/lichess-org/lila/blob/45b5f0cfbbf6c045ad7...

  send = (t: string, d: any, o: any = {}, noRetry = false): void => {
    const msg: Partial<MsgOut> = { t };
    if (d !== undefined) {
      if (o.withLag) d.l = Math.round(this.averageLag);
      if (o.millis >= 0) d.s = Math.round(o.millis * 0.1).toString(36);
      msg.d = d;
    }
    if (o.ackable) {
      msg.d = msg.d || {}; // can't ack message without data
      this.ackable.register(t, msg.d); // adds d.a, the ack ID we expect to get back
    }

    const message = JSON.stringify(msg);
    ...

Which is calculated from how long the server takes to respond to ping messages that the client sends:

  private schedulePing = (delay: number): void => {
    clearTimeout(this.pingSchedule);
    this.pingSchedule = setTimeout(this.pingNow, delay);
  };

  private pingNow = (): void => {
    clearTimeout(this.pingSchedule);
    clearTimeout(this.connectSchedule);
    const pingData =
      this.options.isAuth && this.pongCount % 10 == 2
        ? JSON.stringify({
            t: 'p',
            l: Math.round(0.1 * this.averageLag),
          })
        : 'null';
    try {
      this.ws!.send(pingData);
      this.lastPingTime = performance.now();
    } catch (e) {
      this.debug(e, true);
    }
    this.scheduleConnect();
  };

  private computePingDelay = (): number => this.options.pingDelay + (this.options.idle ? 1000 : 0);

  private pong = (): void => {
    clearTimeout(this.connectSchedule);
    this.schedulePing(this.computePingDelay());
    const currentLag = Math.min(performance.now() - this.lastPingTime, 10000);
    this.pongCount++;

    // Average first 4 pings, then switch to decaying average.
    const mix = this.pongCount > 4 ? 0.1 : 1 / this.pongCount;
    this.averageLag += mix * (currentLag - this.averageLag);

    pubsub.emit('socket.lag', this.averageLag);
    this.updateStats(currentLag);
  };