Many activities that cause blocks to update result in lag spikes, specifically in Realms. Culprits include but are not limited to: planting seeds, breaking wheat, fire spread, breaking logs that are touching leaves, and pistons (cause the worst lag). Taking part in any of these activities usually causes a brief but very sharp lag spike taking frames below 10 fps. Spamming the pistons causes the game to practically freeze. I tested this in SP and found none of the issues occur there... Only Realms