Experiencing server lag (FTB Ultimate reloaded)

[Sadoaz]

I've started a server with some friends (1-9 people, 9 at max), and we are experiencing lag issues.
The message thats occuring in the console is: "Can't keep up! Did the system time change, or is the server overloaded? Running 4130ms behind, skipping 82 tick(s)".

This warning message happens the most when there is alot of people online.

I've have tried allocating more ram to the server (6G), but it seems to still be locked at 2112MB whenever i open taskmanager.
(Internet shouldn't be a problem (500 Up and 500 down).

We are playing on the modpack: FTB Ultimate reloaded, with spongeforge running two plugins (nucleus and luckperms).
The server is running on a old desktop computer that i have.
The specs are:
CPU: Intel(R) Core(TM) i5-3570 CPU @ 3.40GHz.
RAM/MEMORY: 12GB DDR3
Windows 10 home.
Storage: HDD 232GB

 

[Sadoaz]

It has also crashed two with the following message:
"Cause Stack Frame Corruption!
Found frames left on the stack. Clearing them all."

Pastebin from the crash-report:
https://pastebin.com/cZUg0PJQ

 

[grandrolf]

Looks like you have issues with sponge, head over to Sponge community to get help with your issue.

Storage: HDD 232GB

This one I should switch out for a SSD to get better performance.