6, 7 seconds? Few days ago there was a 1 HOUR lag. I am not kidding. All MtGox APIs have lagged a whole frickin' hour behind their trading engine. Now THAT was bad. And I see more than 60 seconds lag nearly every day.

Edit: Seems you updated the post, so now my is obsolete :).

Anyway, their official explanation is that the engine is so slow because when iterating over the whole orderbook, which is huge, they have to check if every offer has enough funds for it to be executed. This is a design flaw and they want to fix it by not letting you to place a order without sufficient funds in the first place.