If you use a setblock clock on a minecraft server, everybody but the player who has the server running gets 5 seconds or so of lag, making chat, snowballs, damage etc. get to be very delayed. You can test this by creating 2 command blocks in the pattern below. The command block on top needs this command: "/setblock ~ ~-1 ~ minecraft:redstone_block 0 replace" and the command block on bottom needs this command: "/setblock ~ ~1 ~ minecraft:air 0 replace"
Configuration (CM= Command block):
[CM]
[AIR]
[CM]