Whenever a new monster or effect appears for the first time in a level, the game almost always locks up for 2 to 5 seconds on my computer. This is triggered when, for the first time, monsters spawn, when items are used, when abilities are used, and even when a treasure box is opened for the first time.
This problem is made worse by the fact that whenever the lockup occurs, I am usually moving. This inevitably causes me to run straight into groups of monsters. It also occasionally causes me to die when surrounded, instead of escaping with shields and careful movement. I also get hit many times more than I should because of these lockups.
I don't get lockups for anything which was previously loaded (including things which caused a lockup). Almost none of these lockups should occur - all monsters and possible effects "should" be loaded and cached when the level is loaded (or earlier).
I think a lot of the lag complaints on this forum might be related.
My first guess would be insufficient RAM to handle the entire system. Java's garbage clean up is one thing, but that sounds a lot more like heavy page swapping.