When your JavaScript takes a long time to execute, it slows down your page performance in several ways:
Network cost
More bytes equals longer download times.
Parse and compile cost
JavaScript gets parsed and compiled on the main thread. When the main thread is busy, the page can't respond to user input.
Execution cost
JavaScript is also executed on the main thread. If your page runs a lot of code before it's really needed, that also delays your Time To Interactive, which is one of the key metrics related to how users perceive your page speed.
Memory cost
If your JavaScript holds on to a lot of references, it can potentially consume a lot of memory. Pages appear janky or slow when they consume a lot of memory. Memory leaks can cause your page to freeze up completely.
How the Lighthouse JavaScript execution time audit fails
Lighthouse shows a warning when JavaScript execution takes longer than 2 seconds. The audit fails when execution takes longer than 3.5 seconds: