If it's performing a request upon closing the tab, only two options are available:

1. it delays closing until it completes, which is a performance hit, or

2. it's performing more website specific background work when I've told the browser not to by hitting the Close Tab button.

Interrupt the request, clean up after the tab.

That's all. The second option is garbage collector. Not sure why the website is entitled to execute any code when I decided to close it.