in the time slider you should be able to select the keyframes you want to scale by holding shift and LMB dragging to select the keyframes you want scaled (the area will turn red, and keyframes yellow)... in the middle there's two arrows next to each other. this, or anything in just the red area, will move the set of keyframes you've chosen. on either side there's a single arrow. these will only scale their position along the timeline, and shouldn't change any values. now you can make your motion slower or faster.

note: scaling frames like this can cause keyframes to lie on fractional frames... ie if a keyframe was on frame 3, after scaling it is possible for it to be on frame 5.64.