When using Minecarts or more complex redstone circuits the entire server crashes. If playing solo, lag is apparent but the server holds up. Add on one or two more players and the lag is extremely apparent.

Timing on redstone circuits is thrown off by multiple people on server. If the circuit has too many components the game will lag and realm will crash. Same for using a minecart on rails with power rails every 8 spaces. Game will lag at each powered rail and if multiple people are on the entire realm crashes. Eventually it reboots to moments before the crash.