The term knot is a measure of speed equal to one nautical mile per hour. In general, sailors consider anything over six knots to be fast.
However, the actual knot count that is considered fast depends on the boat and the conditions. For example, a small sailboat in calm water may only need to maintain four knots to keep moving forward, while a large yacht in choppy seas may have to sustain eight or more knots to make progress.
In short, it really varies depending on the situation. But in general, if you're going over six knots, you're doing pretty good!