I have a server running on 1.12.2 with only SpongeForge and Pixelmon for about 5-6 players on my personal computer.
The issue is that my console sometimes doesn't update with what is going on with the server. For some strange reason I need to have the console window selected and push a button on my keyboard just for it to show anything I had missed. I actively use my computer at the same time it's running but it also happens if i'm completely away with no other program open.
Usually the following warning message appears after pushing a key:
[18:36:47] [Server thread/WARN] [net.minecraft.server.MinecraftServer]: Can't keep up! Did the system time change, or is the server overloaded? Running 3142047ms behind, skipping 62840 tick(s)
This occurs more frequently when more of the players are playing on the server so I assume that it can be my CPU. Even while playing at the same time, I can have small hiccups but less "skipped ticks" happen like on this message:
[00:14:27] [Server thread/WARN] [net.minecraft.server.MinecraftServer]: Can't keep up! Did the system time change, or is the server overloaded? Running 2924ms behind, skipping 58 tick(s)
Let me know if anything else is needed.
Java: Version 8 Update 202 (64bit) (Also have SE Dev kit if that mattered)