"Can't keep up!" message constantly popping up in server console log

[Oceanus5000]

So I've set up a Forge 1.7.10 server for my friends and I, but whenever someone logs in, it keeps saying "Can't keep up! Did the system time change, or is the server overloaded?" in the console, and makes mobs take about 10 seconds to realise someone has hit them, which they then respond to by lagging around.

 

I've made sure my device was plugged in, restarted the computer the server was on, even took out some mods that might be causing lag, but nothing has worked.

 

The server is running on a Macintosh tower that has a 2 x 2.4 GHz Quad-Core Intel Xeon processor, 16GB of RAM, and an ATI Radeon HD 5770 1024 MB graphics card (don't know if the latter really matters, but what the heck). It's running macOS Sierra v10.12.1 as the OS, and I've dedicated 10GB out of the 16GB of RAM I could dedicate to the server, to it.

 

Console Log paste: https://paste.ee/p/l4jae

 

Is there anyway to actually fix this issue? I really don't want to have these issues, nor do I want my friends to have them.

[larsgerrits]

1.7.10 is no longer supported by Forge.