Minecraft short server lags?

I have a Minecraft server network, and recently there are often lags like on the Gomme lobby, which you can fly eg with the double jump or break down blocks, the lags are really always short, what can I do about it?

There are 2 possibilities:

Your own wlan is lagging, vllt use many of you the Internet (Ask your friends how it is with them)
Your server is overloaded with the plugins and worlds - you have to make sure you have enough memory.