Games : Minecraft server is overloaded by one specific player

I run a 1.8.3 vanila Minecraft server.

The problem with it is that it's being overloaded by one specific user (let's call him U) who runs a 1.8.3 vanila Minecraft client w/o any tweaks on Windows.

This is how the server feels before U logs in:

and that's just after it:

The server continues being overloaded and skipping ticks while U is online:

and starts feeling well immediately after U disconnects:

The server on the screenshots is running only U and another player. It may run extra 2 people w/o any changes if U is not online. The odd stuff is only happening in U's client log:

http://xsnippet.org/360574/

The server is being launched like this (it's on a machine that has 5 gigs of RAM):

java -Xmx4096M -Xms4096M -jar minecraft_server.1.8.3.jar nogui