Is this when you're using the brushes or not?

Large brushes with complex shapes put more strain on the computer when your using them.

Also If your using windows try to keep you C: drive (or the partition windows is installed in) with plenty of free space as this increases performance, Filling this drive causes the computer to slow down significantly.

If your not using the brushes i can't see why it would do this, what are you doing when you notice the lag, or is it just in general?