Sadly, the Archer C7 cannot exceed 60Mbit/s without hardware offload when running OpenWRT. I had to replace mine with a Ubiquiti wireless access point and a dedicated pfSense box.
I have performed some simple tests using `iperf` tool on Archer C7 with OpenWrt and it was able to sustain wired network speeds of around 750 Mb/s and wireless speeds (IEEE 802.11ac) of around 300 Mb/s (maybe even more, but I do not remember exactly).