After screwing around in the editor for some time (nothing specific note/time-wise though) the game has huge lag spikes that do not go away after going out of the editor - turning off undo states also doesn't help if they are there already. Mapping without undo states is active does not do any problems (except that you have to be more careful with mapping)
osu! version: 20140814.1 (latest)
osu! version: 20140814.1 (latest)