The cause in plain words, with the fix.
Attach the files or connect the repository. The reply names the line at fault and why it breaks, and the fix touches only what has to change.
Checkout times out when traffic spikes. Logs attached.
The timeout comes from one query per cart line in totals.js1. A forty-line cart makes forty round trips2. One batched lookup3 removes the wait, and a new test fails if it returns4.