Depends on your routing tables. You'll probably find that your PC's encrypted VPN tunnel isn't going via your router's encrypted VPN tunnel, so the router's CPU probably has no effect in this instance. That's my guess, anyway.
As pointed out by @takimata , your Asus is powered by low cost MT7621 SoC (880MHz MIPS). You will NEVER see more than 19 mbps over ethernet (lower using wifi) with openvpn running on the router with AsusWrt or OpenWRT.
Openvpn on Broadcom powered AsusWrt routers should be capable of at least 50 mbps out of the box. eg. RT-AC86u (1.8GHz ARM SoC) is known to support 70 mbps openvpn speeds, and upto 150 mbps when running custom Rmerlin asuswrt builds.
I have a TPlink VR2600 (1.4 GHz IPQ8064 ARM SoC) running OpenWRT with openvpn client. That maxes out at around 65mbps using ethernet.
I recently tested 2013 i5-3xxxm laptop running openvpn client on x86 OpenWRT booting from flash drive. I can get 482 mbps openvpn speeds (ethernet) out of my 518 mbps FTTP connection, because I understand openvpn client can use the AES hardware encryption provided by the Intel processor to significantly improve performance.