At work we have a 100/100mbit internet connection (fiber), "business class" from a very solid ISP. We're 10 people here. Not long ago internet was horribly slow to the point that it took literally a minute to load my usual news site. Ping was up in the 1-2 second range.
Turned out one of my coworkers was downloading the some Windows ISOs from Microsoft.
If we didn't have QoS on the VOIP I'm pretty sure we would have noticed quite quickly.