Commit the offset by manual using sarama library becomes very slow

Question

When I use kafka-producer-perf-test.sh to simulate the producer sending 10,000 messages to a topic 't' with 2 partitions, and use the consumer code mentioned below with dfv1.CommitN set to 3, I observed that when consuming message number 9,748, the sess.Commit() operation becomes significantly slow, taking around 500-1,004 milliseconds. However, when consuming messages from number 9,747 onwards, the sess.Commit() operation takes only around 1-3 milliseconds.

func (h *handler) ConsumeClaim(sess sarama.ConsumerGroupSession, claim sarama.ConsumerGroupClaim) error {
    i := 0
    for m := range claim.Messages() {
    msg := m.Value
    if err := h.f(context.Background(), msg); err != nil {
        // noop
    } else {
        sess.MarkMessage(m, "")
        i++
        if i%dfv1.CommitN == 0 {
            t1 := time.Now()
        sess.Commit()
            t2 := time.Now()
            fmt.Printf("consume cost: %d ms\n", t2.Sub(t1).milliSeconds)
        }
    }
    }
    return nil
}

Why the consume becomes slower after consuming the message number 9747?

Hope the cost time around 1-3ms for each sess.Commit()