Yes it will, but you need to realize that due to how traffic management works, the measured download ‘speed’ will always be recorded at a fraction of what you might have seen before. But that’s actually a good thing, as we’ll explain.
The main goal of traffic management is to ensure low-latencies and a fair allocation of the bandwidth to all clients and processes using the network. This means that allowing any one connection to utilize all the bandwidth is not a good thing. Especially when to do so, it must hog the line for extended periods of time sending very big packets, during which period, other traffic, some of which might be small, but very time-sensitive is held up. This added delay or latency is bad for modern Internet applications, so the traffic manager needs to shape the traffic such that everyone gets a fair shot to get data in or out with minimal delay.
This means that as speeds increase, the shaping progressively limits the maximum size of the packets being sent at any one time such that low latency and fairness is achieved. This impacts the maximum flows any one client can effectuate, and means a speed test will progressively get further and further from the line maximum as line speed goes up. So on 12Mbps DSL, speedtests typically measure 15% lower (or 10Mbps) with traffic management. But a 300Mbps line might only measure 160Mbps, or 53% lower, as allowing the big packets necessary to saturate the line would interfere with other, possibly more time-sensitive traffic. But overall, the traffic can move at line speed when there is no congestion, and multiple connections from multiple devices all get fair access and good throughput at all times.
Technically, the reason is that traffic management in the router encourages the TCP stack of the clients to use smaller Congestion Window sizes (Cwnd), again to ensure fairness both with a clients connections and across all the devices in the network.
So to be able to fully saturate a 350Mbps cable line (without traffic management) the Congestion Widow sizes must grow to be very large, in this example into the low 100’s. But with traffic management, Congestion Window sizes are moderated to a range of 25 to 55, so a quarter to half of what is necessary to fully saturate the whole line from one client. It is these smaller congestion windows that ensure greater fairness for all traffic, and very large packets do not hog the lines causing latencies.
And that’s why on a 250+ Mbps line, the maximum a speed test will ever read will be under 200Mbps, sometimes well under. But that does not matter, as most traffic is way more sensitive to latencies than they are to capacity.
For further info on speed vs latencies, please see our articles on http://evenroute.com/speed