Wiggle effect - can't 'decelerate' / tween the speed between keyframes

I'm using the Wiggle effect to make some icons shake.

I want them to start slow and then for the speed to increase to a crescendo, then slow down again.

I'm just keyframing the speed attriubute and it's just not working as expected.

The speed sort of increases, but then doesn't smothly slow down - it kinda goes crazy at the the 'fast' keyframe and then suddenly slows, it doesn't graduall decelarate as it should.

Keyframgin the 'amount' works fine, but it's the speed I want to control.

I've tried using a wiggle expression instead, using sliders to control the 'frequency' but I get the same issue.

Is there a known issue (and workaround) for decreasing wiggle speed?