"Quicker than one can say Jack Robinson" is a phrase used to suggest something happening very quickly or suddenly. However, there are many other synonyms that could be used to convey the same meaning, such as lightning-fast, rapid, swift, and fleeting. Other common options include instantaneously, in a blink of an eye, like a shot, or in the twinkling of an eye. Each of these options can be used in place of the more familiar "quicker than one can say Jack Robinson," depending on the context and desired effect of the phrase. Choose the right one to express your ideas more accurately and vividly.