Playing in the Pre-1 snapshot in several different seeds and in both creative and survival.

Open shipwreck Map Chests results in a significant lag spike.
Opening multiple subsequent Map Chests in other wrecks causes increasingly longer spikes and about the 4th such chest results in the game not responding.  I have to force quit the game.

It took playing a few different seeds before i made the connection with opening the Map Chests.