You will add frames (length)by slowing down and remove frames by speeding up. If you do equal amounts of both (at differet points :>)
your clip should remain the same length. By expanding the time line to make small changes easier to work with, you could place a node just after the first frame and just before the last frame and bring each to ‘normal speed. But you would not notice this visually (It would be like a subliminal change) To notice it visually I would think that you would need at least 10-15 frames (1/2 second)
I hope this helps.