I recently increased my home internet speed from 100mbps to 400mbps. I had previously been using a QOS down speed of 120 (measured via speedtest.net). With my new down speed I tested again and set the QOS dow speed to 480. At first speed tests showed that I was continuously acheiving this speed. Not long after the speed dropped to approximately half of that ~220mbps.

Changing the QOS setting allows me to achieve the full throughput for a brief period of time (I'm assuming the QOS feature is restarting at this point) and breifly after, say 30 seconds or so the speed is again limited to ~220mpbs. The devices I'm measuring from is a "high priority" device. I've done this test both wirelessly and wired from the main deco.