Countdown freezes on phone browser

URL of experiment:

Description of the problem:
I’m building a novel experiment where participants have to alternate between physically running and solving cognitive tasks. They have to run for a minute at a time, so the experiment has to show a 60-second countdown ending with a beeping noise to let the participants know they have to stop running. This works fine on Chrome on a laptop but the countdown freezes when I try to run it on a phone browser. The console doesn’t show any error messages.

My countdown consists of a list of all the numbers counting down from 60 and time converted to a list index, indexing out one number at a time and presenting it. Is there an obvious reason why this would be computationally taxing? Is there an easier way?

Thank you in advance.

Actually, this is the kind of warning I get (at a late stage in the experiment, so I’m guessing it’s somehow a storage issue):