Safari (like most other Apple-developed apps) aggressively restricts processing in tabs or windows that aren't visible to the user. Apple calls it, adorably, App Nap: https://developer.apple.com/library/archive/documentation/Pe...

The annoying part is that App Nap also does this with plugged in desktops or even worse with servers that don't even have a monitor plugged in. At least it can be disabled systemwide.

Throttling background apps has its place on a desktop, too - leaving aside power efficiency, there are material performance gains to be made when the OS automatically prioritizes foreground tasks. I wouldn't want FCPX rendering impacted by a rogue banner ad on HowToAddLensFlare.co.cz, know what I mean?

I can see how it might be problematic in a headless environment, but for most people most of the time it's probably a useful feature.

Until some unknown stray process starts to eat 100% of your CPU and you just have no tools to find which tab it is. You can kill the process easily tho, but I'd rather fix problem at it's root.