Server lags when player join server.

I have a 4 GB Ram server. I run bukkit with

When a player join the server and his screen showing "Downloading terrain", all players start lags for a moments, the server shows a message like: " [WARNING] Can't keep up! Did the system time change, or is the server overloaded?" and back to normal after he logs in or spawn.
When i checked on "/nolagg stat" the memory used is 412 / 2667 MB.
I don't have any Idea how to fix this. my plugins: