Counting to ten asks you to keep track of numbers at the exact moment you cannot keep track of anything. Breathe out slowly instead and let the breath do the delaying.

What the research says

A meta-analysis of heart rate variability biofeedback found that training slow breathing at a person's own resonance frequency produced moderate improvements in anxiety, depression and performance measures. That is decent support for slow breathing as the anchor. Nobody in that work was compared against counting to ten, so the choice between them rests on the plain point that breath does not need your concentration.

Strength of evidence: one line of research. A single study, or a theoretical account.