Vertices animation with speed graph

Hello! I've been using AE for a few weeks and can't figure out how to smooth speed graph when animating vertices the way it happens with "roving across time" when animating position property (like I drew at 1st image)?

I can do it with three keyframes by moving one at the middle higher (2nd image), but when it is more than 3 keyframes, that gap appears (3rd image).