"[player] moved too quickly!" in the console

What it means

The server checks every player's movement. If someone seems to move further in one tick than should be possible, it logs a warning and teleports them back:

[Server thread/WARN]: Steve moved too quickly! 12.4,0.0,8.1

To the player this feels like rubber-banding: being snapped back a few blocks.

Common causes

• Server lag. When the server falls behind, it receives several movements at once and thinks they happened together. Check the console for "Can't keep up!" and see why your server is lagging.
• Elytra, riptide tridents and fast minecarts can trip it when the server is busy.
• Mods or plugins that give speed or flight without telling the server.
• Hacked clients with speed or fly cheats.

Fixes

1. Fix the lag first. If the server's behind, everything else is a symptom. Lower simulation-distance, pre-generate the world, and look for heavy farms or mods with spark.
2. Allow flight if you use mods or plugins that let players fly: set allow-flight=true in server.properties.
3. On Paper, Purpur and Spigot, the thresholds are moved-too-quickly-multiplier and moved-wrongly-threshold in spigot.yml. Raising them slightly helps if legitimate fast travel keeps being pulled back, but fix lag first.
4. If it's one player and the server isn't lagging, they may be cheating. An anti-cheat plugin will catch it more reliably than this warning.