Teleporting a player many times in a short period of time (e.g. once per game tick) queues network packets, causing lag.

Mainly the packets sent from client to server pile up. ("tx" in debug screen)
Their amount does not grow proportionate, meaning that teleporting one player with a steady frequency (20 times a second) makes the amount of packets sent grow more and more over time. For example 5 packets are sent without teleporting, 10 after one seconds of teleporting, 20 after two second, 30, 40, 50,...

The player's position might get reset or the player might even get kicked for "flying" because the connection has such a delay.

This only happens in multiplayer.