In a browser or Node, a single line of work, called a thread, takes jobs off a list and finishes each before starting the next. While a job runs, nothing else can: no clicks, no animation, no other requests. A job over 50 ms in a browser is called a long task.
An agent's code works on its small test data and passes everything, and then it sorts 200,000 rows inside a click handler and the page freezes. No test notices a freeze. In Node it is worse: one slow job stalls every other person's request on that server.
Say it to your agent
“This click handler sorts all the rows on the main thread. Move the sort to a Web Worker. Then record PerformanceObserver longtask entries before and after with 200,000 rows, and show me that none are over 50 ms.”