Why does free download manager slow when not active tab

I have this on my phone. When it is downloading and I am on another site,like now I am here on reddit ,it slows down . It's still open in the background, not even background, another tab probably better explains. It's not the active tab. So why does it slow ?

Also I have FTTH with up to 500 MB/ps download speed on my wifi but FDM top speed is one MB per s ???