IPv6 WiFi bad performance

[ccc1]

Asus router (RT-AX1800U) already updated with latest firmware.

1) with IPv6 enabled - Wifi connection seeing high packet retransmission, but wired connection no issue with performance. If surfing site (site has IPv6 address) on wifi e.g. www.google.com, ipv6.google.com, www.youtube.com loading very slow with some parts of page not loading. If surfing site (site has only IPv4 address) on wifi, no problem with browsing. IPv6 connectivity fine working (ISP with another router from another vendor tested IPv6 is good so not issue with ISP) but just cannot browsing performance is very bad for some major sites (slow and hang and high retransmission) on ASUS router with IPv6 enabled.

2) with IPv6 disabled - Wifi and wired connection all working fine.

Tried tuning MTU and MRU for WAN values down until 1280 (minimum for IPv6 to work) still same sometimes improvement for some major site but does not fix for all above major sites.

Suspect issue is with ASUS router IPv6 implementation does not support path MTU discovery thus causing packets to be dropped and thus high retransmission.

[Falcon_ASUS]

@ccc1
Here we mainly discuss the issues/questions related to ASUS laptop.
For your Router questions, I suggest you contact our local call center to have further assistance.
Please refer to below and find the link of local call center.
https://www.asus.com/support/CallUs# 
Sorry for any inconvenience it may be caused. 

[SiriusDark]

So I know below the ASUS moderator said this is for issues related to the ASUS laptop but I wanted to let you know that this isn't isolated to your model of router. I have the RT-AX88U and it has the same issue. I determined that there was a routing issue because IPv6 ping tests were showing "No route to host" meaning there was a messed up IPv6 routing table. Turning on AP mode and stopping the ASUS from routing any traffic did resolve the issue. I have a case open with ASUS support about this and have informed them. As of right now I have no further workarounds.

[ccc1]

Thanks for sharing @SiriusDark , my IPv6 ping test is working so not not a routing issue and even browsing can work for some sites but not for other more major sites. Issue is more towards performance of WiFi is really bad compared to wired. Hope Asus has a solution, otherwise if none, will try to test AP mode like what you suggested but need to add another router in front to handle the PPPOE and DHCPv6-PD stuff.

[Falcon_ASUS]

@SiriusDark
I apologize deeply for being unable to assist you. 
If your product model is RT-AX88U, you can report your problem from here.
https://rog-forum.asus.com/t5/gaming-routers/bd-p/NW_Routers  
Sorry for any inconvenience it may be caused.