Strange throttling or wifi slowing with DSL-AC86U

[zeiphon]

I have a brand new DSL-AC86U which appears to be throttling or otherwise slowing the wifi connection only to a windows laptop. (I have no other PCs connected or at hand, just android phones)
Firmware version: ASUS 3.0.0.4.386_39648
The VDSL connection is 25mbit/5.
Multiple android phones have been tested to achieve ~24mbit, 2.4Ghz or 5Ghz.
Windows laptop: Begins at ~4mbit, quickly slows to 600kbit-1mbit. Upload remains fine at 5mbit. Both 2.4 and 5Ghz.

Here's the interesting thing: If I connect to an external VPN, the laptop achieves 12mbit+. This has been tested with multiple VPNs.
If I hotspot an android phone on 4g, the laptop achieves >40mbit.
If I hotspot an android phone that is connected to the DSL-AC86U (newer android phones can act as an AP + client simultaneously) then the previous 600kbit-1mbit speed returns, even though the phone itself achieves 24mbit.

It seems to me like the router is performing some kind of traffic inspection or analysis perhaps (and hates my laptop), or maybe there's a setting causing this issue I haven't discovered.

QoS is disabled. Enabling/disabling has no effect.
IPTraffic is disabled. Enabling/disabling has no noticeable effect.
URL/Keyword/Network services filters are disabled.
NAT Acceleration: Auto: CTF (Cut Through Forwarding) is enabled. (this wasn't the case in gnuton merlin, see below)

I have switched to gnuton's merlin for DSL-AC68U and the exact same issue occurs. However next to 'NAT Acceleration' the message "NAT Traffic is processed by CPU" appears. In the sysinfo tab, HW Acceleration is disabled, with message "incompatible with:" but no actual listed service (again IPtraffic is disabled. I enabled it, and it appeared next to this message, but disabling it again still results in HW acceleration being disabled with no listed cause).

I am very much out of ideas with this so any help would be appreciated! Let me know if any other setting values etc would be helpful.

Thanks!

 

[zeiphon]

Alright I think I've found the solution to this one.

WAN > Edit PVC > Enable 802.1Q.

Why the hell does VLAN packet info affect wifi performance for only a windows laptop? At least it works..

 

[BK303]

Thanks for sharing, I have also been struggling with what impacts my RT-AC86U throughput, but different circumstances. Always helpful to see what settings others find that impacts throughput.

Just some thoughts....
I wonder if 802.11Q VLAN or 802.11p (PCP priority bits) are somehow enabled on your laptop's WiFi adapter? Maybe the laptop is not setting those bits the same way as other devices you tested, so there was some undesired interaction with the Router's 802.11Q processing. If using Windows on your laptop, depending on driver being used, you may find some related settings under Control Panel > Network Connections > WiFi Adapter > Properties > Configure > Advanced, look for 802.11Q or VLAN setting. But if I recall correctly, Win 10 removes some VLAN controls from that properties GUI, and you can't even see them anymore unless you have special driver and use powershell. An alternative way to probe deeper could be to use WireShark on a LAN device to examine the 802.11Q bits received from the laptop compared to traffic sent from another LAN-side device.