Percieved issues on MineOS, minecraft, and memory comes around with almost regularity.
Please read the following posts:
The &TLDR of it is that the numbers shown in the WebUI is at best to be considered as an estimate, and may not reflect real memory usage.
I can see that you have reserved 17 873 MB RAM in total for your servers. Is this below your total physical RAM available? Since you also have 6 426.35 RAM available according to your WebUI, I guess it is. This means that RAM is most likely not your issue.
Your load average is also low, in the short timespan (yellow), and stable at 1 in the long timespan (blue), and above one and aproaching 1.5 in the mid timespan (orange). The whole umbers represents one CPU/Core, so if you have one CPU with one core available you want your load average to be bellow 1. Most CPU’s now come with several cores, so if you have 2, average should stay below 2, and so on.
You compare MineOS (and your lag there) with other Management systems, and barebone management. Is your comparizon correct? Did you run the exact same servers, with the exact same setup, number of users, available plugs/mods, same number of active servers, on the exact same hardware? For each of those you answer “no” to, I am sorry to say thet your comparison invalidates itself.
MineOS uses the memory limits you define to run java just like you would manually (like this: java -Xmx1024M -Xms1024M -jar minecraft_server.1.11.2.jar nogui), but in a screen session (https://www.mattcutts.com/blog/a-quick-tutorial-on-screen/). All the system information MineOS then gives is repported by the system. MineOS it self do not really control resource usage after your minecraft jar has been started. MineOS relies on the OS and Java to do this.
I think this has two solutions:
Optimize your minecraft servers, and their plugs, and number of users available at the same time.