Server Lag

I understand that in all games there is LAG....

But being new to minecraft I am not sure how the "server" is hosted.

I have hosted many game servers, such as battlefield 2, battlefield 3 and Runescape private servers.

Is minecraft the same? if so who is the Host and what can be done to improve the Lag?

It only seems to start lagging really bad when there are 15+ players on the server.

Nitrado is the host of the server. Nitrado is a company that hosts minecraft server and radu is paying Nitrado to host the server

Well I don't lag on RuneCraft, it may be your connection. ?

Sometimes I lag and sometimes I don't lag. When you restart the server though, the lag dies down. After 1 hour or so goes by after the restart, it starts to lag like crazy and your blocks start disappearing after you place them.

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I always thought Radu hosts the server from home. I guess I am wrong...

Thank you,
builder3128