it lags only if ur on another tab or u have alot of activity, notice on the bottom right hand corner when ur making a track it tells u what percent ur activity is on. the lower the less lag. but when u render it all it should clear the lag
If you're working on only a small part of your arrangement and you only need to hear that, deactivate all the tracks that you don't need on the time-line. In my experience, this helps a lot.
Wrong board.