[WARNING] [Minecraft-Server] Can't keep up! Did the system time change, or is the server overloaded?

No.
No the time didn't change.
No the server is not overloaded.
There's no-one frickin using it.
No processes other than MC.
And no users because I re-started it.
Version 2.9.2.4

top - 12:23:31 up 12 days, 19:51,  4 users,  load average: 0.57, 0.46, 0.42
Tasks: 122 total,   1 running, 121 sleeping,   0 stopped,   0 zombie
%Cpu(s): 13.9 us,  0.4 sy,  0.0 ni, 85.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:  12286436 total,  5003584 used,  7282852 free,   388952 buffers
KiB Swap: 16756732 total,        0 used, 16756732 free,  2731256 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND
 7137 kim       20   0 4369m 1.4g  22m S  58.6 12.0  86:01.56 java

No one else on the server.

I stopped the service and re-started it.

As soon as Galacticraft loaded, I got this warning.
What the hell?

Can someone please modify that warning to give some more indication as to why it occurs?

Please?