I'm running an older minecraft server version 1.7.10, because of very good mods that mostly are only available for 1.7.10.
Over time, the minecraft server application received a lot of improvements performance wise. Like better utilizing CPU, RAM or optimized netcode and use of new - more efficient - Java version.
At a certain point in game - especially if a lot of CPU hungry entities are running - the server can't keep up and lags appear. After a restart, it runs for a while, but that point where it gets unstable will be reached. I think it's a limit of the minecraft version itself and not of server resources.
IIRC a lot of code and overall structure of the game has changed after 1.7.
Is there a way to use the last minecraft version without porting each 1.7.10 mods?
Like providing interfaces or adapters the old mods speak to, but are mapped to latest mc.