When you load into the world initially it starts at 20 ticks per second. However, after using spreadplayers and pushing everyone out thousands of blocks, the world tick rate drops to about 3 ticks per second. Even after everyone is teleported back the tick rate doesn't return to 20. Even if you teleport back to spawn, it won't get rid of the effect until you reload the world.

It doesn't matter how many people are in the world. It effects singleplayer as it does multiplayer.

Steps to reproduce:

1. Generate or load a world (superflat, normal, doesn't matter)
2. Build a redstone clock at spawn. It'll work at a normal speed.
3. Use the command '/spreadplayers ~ ~ 0 5000 @s'
4. Once you land build another redstone clock. It should tick very slowly now.
5. Reload the world. The clock should operate at normal speed.

Effects on the world:

• The player seems unaffected by this. Hunger goes down as normal, and physics don't change.
• Eating, however, is slowed to a painful crawl.
• Redstone is slowed down massively.
• Animals and other mobs are in slow motion
• Furnaces/Smokers/Blast Furnaces take more time to smelt anything