Asuswrt's OpenVPN implementation does not support IPv6, only IPv4. So when using an OpenVPN tunnel with a dual stack provider, all your IPv6 traffic will still directly go through your ISP and not through your VPN tunnel, which is why it will be able to run at your ISP's rated speed. For OpenVPN, the RT-AC87U can hit around 20-30 Mbps with AES-256-CBC, or about 40-60 Mbps with AES-128-CBC (speed might vary a bit based on other factors).