In the current snapshot, something seems odd with the memory usage. The used memory is rapidly climbing up to 1.5gb and then dropping back down to around 200mb. It's doing so for me at around 100mb and seems to be related to the frame rate as recording it with Fraps which ties the framerate to 60 causes it to slow down slightly compared to letting the game free run around 200fps.

This was taken on a multiplayer server with a fresh world with only 2 people on.

Video of the memory usage: http://www.youtube.com/watch?v=xSE9frANa-Q