Tcp needs packets to be dropped quickly to get the feedback that your link can't handle the speed. Without that, it will appear that the speed of the network is

Tcp needs packets to be dropped quickly to get the feedback that your link can't handle the speed. Without that, it will appear that the speed of the network is fluctuating (as the buffer fills it appears your network is faster than it is, when the buffer is full it will start dropping packets which causes tcp to back off which causes the buffer to drain, now if tcp starts sending data before the buffer is empty you'll get a different apparent speed to tcp) and it is that fluctuation which causes the issues.