I've been running a vanilla server with my friends on 1.14.3 for a while with never a hint of lag or anything. I have gigabit download speeds and consistent 50Mbps upload speed. PC has 16GB of RAM with me launching the server in cmd with this command: "java -Xmx4096M -Xms4096M -jar forge.jar".
Exactly the same command as the vanilla server just a different .jar file.
The vanilla server had the memory usage slowly cliumb to its cap then 'refreshing', dropping down to around 500MB and repeating.
The Forge server is CONSTANT 600MB and i believe this behaviour to be the cause of the lag in the server.