Performance
Lag has more than one cause, and the fix depends on which one you have. The server can be slow to calculate the world, short on memory, or fine while the connection to a player is not.
The article below shows how to tell them apart with what the panel already measures. The guides next to it go deep on each cause.
Lag that only appears while people explore new land has a fix of its own, pre-generating the world, with its own article below as well.
Performance articles
The two numbers to watch
TPS tells you whether the server keeps up: 20 is perfect. The ping in the player list tells you how far a player is from the server. A low TPS affects everyone and is fixed on the server. A high ping with a good TPS affects only that player and is a matter of distance or their own connection.
Most lag on small servers comes from exploring new land, a large farm, or a heavy modpack on too little memory. The article below shows how to tell which one you have.
Delivery: most servers are handed over instantly and run within seconds of payment, the rest follow within 60 minutes during the day and within 12 hours at night.