For hardware, I'm using ASRock Q1900B-ITX J1900 motherboard with 16Gb of RAM. I loaded Ubuntu 18.04.2 with Java and the vanilla Minecraft 1.14.2.
Right after starting the server, I get "can't keep up! Is the server overloaded? Running XXXms or YYY ticks behind." I allotted 4Gb of RAM heap size.
What am I doing wrong?