How fast can a pattering heart beat?  As fast as water can drip before it starts dribbling. You can’t quantify that transition, but you know it when you see it.

Other speed similes use a viper to convey quickness: one by Williamson and another by Christopher Logue.

‘His heart beat as fast as waterdrops drip without dribbling.’

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 147

Photo credit: Myriams-Fotos at pixabay.com