Question Asus ZenWiFi AX (XT8) cuts download speed by at least half with QoS ?

[markm75]

Asus ZenWiFi AX (XT8) cuts download speed by at least half with QoS.
Without QoS it's still 17-20%, xfinity comcast?


For months i've noticed that my download speeds with comcast were no where near 800 (and now i have gigabit as well), always around 450 mbps or so. I assumed it was an issue with comcast and even had techs out.

Today i tried something, i took the router out of the picture and connected directly to my pc, i was around 1025 mbps every time, no issue.

So then i reconnected tested again, still 450. I looked in the settings and realized i had qos on, i turned it off, now the speeds are more like 850 to 900 (especially if you use the tool in the router admin area), but can avg 799 or so when using online checkers (the same checker that said 1025 direct connect).

I then tried toggling other settings in the router and couldnt find one to give back that missing ~17% or so.
When i did the 2nd router test after qos was off, i checked traffic for wifi and other things and found it was minimal or none, so that wasnt part of the pull.

Anyone else noticed this issue, or found a work around, or is the qos halving normal, is the 18% overhead without qos enabled also normal? (i also tried other ethernet cables that go from the router to the cable modem to no avail, im using cat6 as well).

Thanks in advance

 

[bill001g]

I would factory reset the router. Then set only the admin and wifi passwords maybe turn off wifi admin and wps if those are on by default.

The key reason QoS cuts the speed so dramatically even with no rules set at all is the cpu must do that function as well as the NAT. The cpu in routers is very small. Almost all routers now contain hardware based NAT but when this is used the traffic in effect bypasses the CPU. Nat is a actually very processor intensive since it much insert the public IP and the recalculate the header checksums.

This pretty much means that a lot of fancy features on modern routers can not be used if the cpu chip must see the data. You should see how bad it is if you use VPN, many routers are lucky to do 30-100mbps.

If it is still slow with all factory settings it is hard to say. The highest you can see is in the 940mbps range. This is the 1gbit minus the overhead used for transmitting the mac addresses. The router itself can exceed this rate because it has a cable/fiber that is actually running faster than 1gbit so any overhead is hidden in that.

 

[markm75]

I would factory reset the router. Then set only the admin and wifi passwords maybe turn off wifi admin and wps if those are on by default.

The key reason QoS cuts the speed so dramatically even with no rules set at all is the cpu must do that function as well as the NAT. The cpu in routers is very small. Almost all routers now contain hardware based NAT but when this is used the traffic in effect bypasses the CPU. Nat is a actually very processor intensive since it much insert the public IP and the recalculate the header checksums.

This pretty much means that a lot of fancy features on modern routers can not be used if the cpu chip must see the data. You should see how bad it is if you use VPN, many routers are lucky to do 30-100mbps.

If it is still slow with all factory settings it is hard to say. The highest you can see is in the 940mbps range. This is the 1gbit minus the overhead used for transmitting the mac addresses. The router itself can exceed this rate because it has a cable/fiber that is actually running faster than 1gbit so any overhead is hidden in that.

Thanks, yeah i figured some sort of overhead, i think this am's test was around 800mbps (out of the 940 theoretical max of the full 1gbit). I didnt have time to try a few things this am though.

I did have vpn turned off on my original tests, i thought wifi was off but maybe not, ill have to try the reset or disabling wifi for sure. This am vpn was on (wireguard). ill try to pay attention to cpu use on the device next time as well.

on the qos, im surprised noone else seems to mention it being on and causing such a dramatic loss with this one at least.

 

[bill001g]

It is rare for most people to even know that router have a asic based nat function. The nat being done purely in dedicated hardware is vastly faster than cpu instructions. Almost all routers have it so you do not see it mentioned in the feature lists.

This is/was the issue with third part firmware. The driver for this function is not public domain so they can't distribute but you can link it yourself for many routers.

In addition only a small number of routers have AES encryption instructions so vpn runs rather slowly

I am not sure why the router would bottleneck. Many similar routers that have 2.5g port or faster lan and wan ports can easily get speedtest numbers that exceed 1gbit.

 

[tony stewart]

The Asus ZenWiFi AX (XT8) has a shockingly low download speed when QoS is activated. While QoS is intended to give traffic priority, some devices may accidentally experience speed throttles. To check if performance improves, think about changing the QoS parameters or turning it off for a while.

 

[das_stig]

Replace stock firmware with custom from https://www.snbforums.com/forums/asuswrt-merlin.42/

He is an approved developer by Asus and use their firmware images to create better version, adding missing features and improving others, QOS being a major improvement.

 

[bill001g]

Replace stock firmware with custom from https://www.snbforums.com/forums/asuswrt-merlin.42/

He is an approved developer by Asus and use their firmware images to create better version, adding missing features and improving others, QOS being a major improvement.

I am not so sure this fixes much. This is a very common problem on almost all routers. The key issue is to do QoS the cpu must see all traffic. This means the CPU chip must also do the NAT function which normally is handled by special circuit that bypasses the cpu chip. Although you can make the QoS code more efficient it still is software running on a cpu rather than being asic based hardware processed.

I kinda is mute issue. You only need QoS when you have queues. If you have data being queued on a gigbit internet connection you have a much larger design issue than simple QoS will fix. It really is only useful on slower internet connection, like under 100mbps. At those speeds you can still use CPU based function and not slow the connection down.