When note moves in the interval more than 2nd, human ear catches it as structure rather than passing note within a phrase. If the last note before the jump or first note after the jump is an avoid note, it stands out and destroys tonality. Slower the tempo, worse the effect.
Here is an example of V-I phrase:
Here is an example when avoid note S4 sticks out and bothers the tonality:
However, if avoid note is followed by a valid note in half-step, the avoid note will act as an approach note. While it still catches ear, the unity of the tonality won't be destroyed as bad: