Keep in mind that the AC86U has a newer 64bit processer. So it's not strange that you should see higher performance with this router compared to older models.
RT-AC86U: Broadcom BCM4906 (1.8 GHz, 2 cores)
RT-AC68U: Broadcom BCM4708A0 (800 MHz, 2 cores)
RT-AC3100: Broadcom BCM4709C0 (1.4 GHz, 2 cores)
Both the AC68U and the AC3100 do not support AES Accelleration, while the AC86U does it. As a result if you would run an OpenVPN client on the router, the speed would not be more than ca. 40 Mbit/s on those two older modells, while I'm getting 240-250 Mbit/s with OpenVPN on AC86U.
Also keep in mind the age of the routers.
The RT-AC68U was released in October 2013 (now four and a half years old).
The RT-AC3100 / RT-AC88U was released in October 2015 (now two and a half years old).
The RT-AC86U was released in August 2017.
RT-AC86U: Broadcom BCM4906 (1.8 GHz, 2 cores)
RT-AC68U: Broadcom BCM4708A0 (800 MHz, 2 cores)
RT-AC3100: Broadcom BCM4709C0 (1.4 GHz, 2 cores)
Both the AC68U and the AC3100 do not support AES Accelleration, while the AC86U does it. As a result if you would run an OpenVPN client on the router, the speed would not be more than ca. 40 Mbit/s on those two older modells, while I'm getting 240-250 Mbit/s with OpenVPN on AC86U.
Also keep in mind the age of the routers.
The RT-AC68U was released in October 2013 (now four and a half years old).
The RT-AC3100 / RT-AC88U was released in October 2015 (now two and a half years old).
The RT-AC86U was released in August 2017.