ASUS RT-AX1800U - working IPv6 but terribly slow

[vykeer]

Hello!
I've been fighting with this issue for a few days, so I'm writing here, maybe somebody can help... I've got ASUS RT-AX1800U router, and that's what happens:

- when I set up tunnel directly in Windows 10 (as instructed in Example Configurations) with forwarding of protocol 41 enabled in my router configuration - everything works fine on that specific computer - it is quite fast, IPv6 pages are loading, pings are working, IPvFoo extensions shows '6' - that's great

- when I disable port forwarding on my router and use its internal IPv6 feature with routed /64 configuration, a mess happens... IPv4 works perfectly good, but in case of IPv6 - addresses are pingable, DNS resolves IPv6 addresses correctly, but speed of IPv6 sites and other services is horrible - 20kbps, max 200kbps.. yes kilobits per second.. something is downloading but I feel like in times of modems - even Google captcha can't properly load. It happens on every device connected to my router's Wifi.. Windows, Linux, MacOS

Does anyone suspects, what could be wrong?

[vykeer]

Anyone.. ?