Lag, in its simplest terms, is impaired functionality, due to many different factors that cause strain on a computer's processor (in this case). Lag can be reduced in many different ways, often by fixing what causes the lag in the first place. But keep in mind that the stronger the computer is, the less lag it will go through, and vice-versa.
Piano from above lag
In crash part such as Bad apple!! or some of the strong crash part
Do not load large notes such as millon notes