Retiming operations act on all of the layer’s parameters, including its visibility keys. Applying a Speed Multiplier with a value of 2 reduces a layer’s screen presence by half.
But it’s possible to make an exception for this specific type of key in order to keep the same visibility on screen, which can be convenient when keys are placed according a precise rhythm or events in the animation.
To do so, you just have to uncheck the Retime Visibility option, checked by default:
Here you can see what happens in the dope sheet when this option is checked or not, while changing the Speed Multiplier: