There are a lot of ways your server could be lagging, this article can help you figure out why its lagging and possible solutions.
Reasons for lag:
Too many players and not enough RAM.
Too many mobs/items and not enough RAM.
You are too far away from where your server's machine is based.
Too many worlds running on your server without sufficient RAM.
Errors are spamming in your console.
Too many plugins running on your server and not enough RAM.
Things to do first:
Reduce Your View Distance
We recommend that you reduce the view distance of your server from the default 10 to a value between 6-8. This can be edited through your server.properties file, you can find this by going to Management->File Management on your control panel.
Clearlag is a plugin that helps with removing mobs, items that could be causing lag within your server. We recommend this plugin for any server.
A guide on installing plugins can be found here.
Once you have ClearLag installed you can run the command "lagg check" to see how many entities you have on your server
If you are running a small server anything over the value "500" may possibly be the reason for your lag.
If you do have a high number of entities, run the command "lagg clear" and "lagg killmobs". This will remove the lag causing entities from your server i.e. items on the ground and mobs.
The more online players and plugins you have on your server, the more RAM your server will be using. If you have a lot of plugins and not a lot of RAM, this can cause your server to overload and have random lag spikes and/or crash.
The best thing to do in this case is to limit the amount of plugins on your server. If you would like to keep all of your plugins, and have tried all of our other steps, we recommend upgrading to a more powerful hosting solution.