There isn’t one universal “fastest possible shutter speed.” In theory, the limit is set by how briefly you can expose the sensor while still collecting enough photons to form an image. In practice, shutter speed is limited by the shutter design, sensor/electronics, timing accuracy, and especially available light.

The main benefit of a faster shutter is freezing motion: capturing very fast events in a tiny slice of time, such as impacts, flying particles, or sports action. Extremely short exposures are also used in scientific and industrial imaging.

A practical tradeoff is light: every time you make the shutter twice as fast, you need roughly twice as much light (or higher sensitivity) to keep the same exposure. That quickly becomes the real limiting factor.

Also, “shutter speed” is exposure duration, not an instant on/off switch. Mechanical shutters take time to open and close, so their quoted speed is an effective exposure time rather than a perfectly rectangular pulse. Electronic and specialized machine-vision cameras can achieve much shorter effective exposures than typical consumer cameras.