If you spawn in the middle of the ocean, the game will lag significantly more than spawning on land. Below is a jfr report made right after first entering a world using the seed -627081940333231321.

Steps to reproduce: 

1. Restart Minecraft
2. Create a new world with the seed -627081940333231321 (which spawns you in the middle of the ocean)
3. Notice that the game is a bit slower than usual