At a rate of knots
Definition 1 of 1
Pro Tip 1/3Sailing Origin
The phrase originates from nautical speed measurement, 'knots'. It implies something moving fast. 
The rumor spread at a rate of knots through the school.
Pro Tip 2/3Informal Tone
A casual phrase, not used in formal writing. Best for informal conversation or creative text. 
News about the concert was spreading at a rate of knots.
Pro Tip 3/3Emphasize Speed
Use when emphasizing the surprising or notable speed of an action or event. 
The new product's popularity grew at a rate of knots.