When you use the command /spreadplayers and the spreaddistance: float is a big number (5000 for example) makes the world slows down up to 6 times, in a medium-high end computer. In slower devices should be even slower. Once the world is restarted it works fine again

How to reproduce:
Open or create any world with cheats enabled
Execute the following command 
/spreadplayers 0 0 5000 10000 @a
You will get teleported to a random location in the world in a 10000 radius and to y=255 (another bug related to the same command so recommended to use the command in gamemode creative)
Once you are in the ground trying to mine any block or make any action you will clearly see that it takes a lot to do.
Once the world is closed and reopened it work fine again

Expected behavior:
Do not lag the world and tp a player to a random location into the highest block that is not air