What device in a switcher is used to produce transitions at different speeds?

The fader bar is the device used in a switcher to produce transitions at different speeds. This control allows the operator to adjust the rate of the transition from one video source to another, enabling smooth and gradual changes between scenes. By moving the fader bar, transitions can range from quick cuts to slower, more visually pleasing fades, giving operators creative control over how content is presented.

Other devices mentioned, like the cut button, typically execute immediate switches without varying the speed, while a mixing desk is primarily used for audio rather than video transitions. The transition lever, while it may suggest a similar function, is not commonly referred to in terms of controlling variable speed transitions. Hence, the fader bar is the most relevant and effective tool for achieving varied transition speeds in a switcher setup.