Singleplayer Skipping ticks

[19:16:21] [Server thread/INFO]: [Wullepa: Set own game mode to Spectator Mode]
[19:16:21] [Client thread/INFO]: [CHAT] Your game mode has been updated
[19:16:34] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3069ms behind, skipping 61 tick(s)
[19:16:45] [Server thread/INFO]: [Wullepa: Set own game mode to Creative Mode]
[19:16:45] [Client thread/INFO]: [CHAT] Your game mode has been updated
[19:17:39] [Server thread/INFO]: [Wullepa: Set own game mode to Spectator Mode]
[19:17:39] [Client thread/INFO]: [CHAT] Your game mode has been updated
[19:17:52] [Server thread/INFO]: [Wullepa: Set own game mode to Creative Mode]
[19:17:52] [Client thread/INFO]: [CHAT] Your game mode has been updated
[19:18:00] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 3792ms behind, skipping 75 tick(s)
[19:18:13] [Server thread/INFO]: Saving and pausing game...

Why are ticks skipped?