I have an XR500 at the latest firmware version (2.3.2.40). I noticed that when I have QoS enabled, my download and upload speeds are slightly lower than normal. I have a fiber connection, and with my PC directly connected, or if I'm connected through the XR500 with QoS disabled, my download/upload are at 480/118. However, if I have QoS enabled, I'm seeing a 2-3% reduction in speed, taking me down to roughly 470/115 depending on the test.
I realize this is a small reduction, but I was curious to know why this is happening. Is there an additional bandwidth buffer reserved by the router any time QoS is enabled, irrespective of anti-bufferbloat and bandwidth allocation settings?
I've tried testing QoS enabled with Anti-Bufferbloat at 70/70 on Always, When High Priority Traffic Detected, and Never, and have also reset Bandwidth Allocation to defaults with with Share Excess option checked. Additionally, I unchecked the DumaOS classified games from Traffic Prioritization and removed all of my additional ports I had added for games not covered under the DumaOS games list. Testing under these conditions produces the same output as when I have everything customized the way I prefer it.
This is mostly called overhead with QoS. When enabled there will be a small portion of QoS that will show a bit lower speeds. If your getting to speeds with QoS disabled and just under with it enabled, then QoS is working correctly.
Thanks for the input. After performing further testing, I realized the download and upload bandwidth set under the anti-bufferbloat options actually dictates how much bandwidth the router allows, even before the anti-bufferbloat settings are taken into consideration. If I list it as 200/50, for example, my speedtest reflects just slightly under those numbers. If I list it as 500/125 (what I'm rated for by my ISP but I don't actually get these speeds even when directly connected to the fiber ONT), I get the same 480/118ish numbers as when QoS is disabled.
One thing I should mention for anyone who might find this through google at some point - Make sure that if you're setting your bandwidth speeds over your speed test results to allow the router to give you your full bandwidth that you take this into account when setting the thresholds for anti-bufferbloat so that you're leaving enough wiggle room for priority devices.
