I just did a short test of running WireGuard on my ASUS RT-AC86U router, following the instructions I found in the Smallnetbuilder forum.
Got it finally working with a small changes, to the config and did some speed tests to compare it to my previous OpenVPN speed tests done on the same router.

Technical information:
Router: ASUS RT-AC86U
Firmware: Merlin 384.5_alpha3
VPN Provider: AzireVPN
Speed test server: Stockholm – NORDUnet A/S

As you can see below, the speed test results are a bit over 400 Mbit/s both downlink and uplink. (regular ISP speed ca. 950 / 950 Mbps)

At the same time while testing the max speed with Speedtest.net, the CPU was working up to 100%, using both cores simultaneously. Something which isn’t possible when using OpenVPN on the router.

Looking good so far. WireGuard can use multi-cores in the CPU, and OpenVPN can’t. Hoping for more VPN providers to implement WireGuard soon.
At the moment (April 2018) I think only Mullvad and AzireVPN do support WireGuard.

Update: 2018-09-15

I wrote an instruction about how to install and run WireGuard on your Asus router. See the link below.

• How to install and run WireGuard on an Asus router

Update: 2019-02-02

New speedtests with the latest version of WireGuard and connected to Mullvad’s servers in Stockholm/Sweden, achieving partly over 600 Mbit/s.

Connecting to host speedtest.serverius.net, port 5002
[  5] local 10.99.XX.XXX port 57217 connected to 178.21.16.76 port 5002
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.01   sec  54.4 MBytes   451 Mbits/sec    0   2.71 MBytes
[  5]   1.01-2.00   sec  73.6 MBytes   623 Mbits/sec    0   2.72 MBytes
[  5]   2.00-3.01   sec  77.5 MBytes   649 Mbits/sec    0   2.72 MBytes
[  5]   3.01-4.00   sec  76.2 MBytes   643 Mbits/sec    0   2.72 MBytes
[  5]   4.00-5.00   sec  76.2 MBytes   637 Mbits/sec    0   2.72 MBytes
[  5]   5.00-6.00   sec  69.5 MBytes   584 Mbits/sec    3   2.00 MBytes
[  5]   6.00-7.00   sec  63.8 MBytes   534 Mbits/sec    5   1.52 MBytes
[  5]   7.00-8.01   sec  47.6 MBytes   396 Mbits/sec    1   1.14 MBytes
[  5]   8.01-9.01   sec  41.0 MBytes   343 Mbits/sec    0   1.21 MBytes
[  5]   9.01-10.01  sec  44.8 MBytes   376 Mbits/sec    0   1.26 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.01  sec   625 MBytes   523 Mbits/sec    9             sender
[  5]   0.00-10.01  sec   623 MBytes   522 Mbits/sec                  receiver