C7 v2 with openwrt will always be inferior to OEM firmware in performance, but even 73 Mbps does seem low.
Does the laptop have 802.11ac 2x2 or 1x1 wifi card?
Does it report 867 Mbps wireless link speed to the C7 if it has a 2x2 card?
When you use iperf3 to test speed from one PC to another PC via the C7, using multiple streams instead of single stream may yield faster speeds. eg. add -P 10 to iperf3 command on client.
The BT Home Hub 5A modem-router uses same QCA9880-BR4A 5 GHz radio as Archer C7 v2, but has different slower 550MHz SoC (Lantiq). In access point mode, you can see recent iperf3 test results posted here
https://forum.openwrt.org/t/18-06-4-speed-fix-for-bt-homehub-5a/23643/23
Single threaded speeds 'upto' 130 Mbps, and multi-thread return 'upto' 167 Mbps for 19.07.0-rc2. These are poor and less than half what OEM firmware could achieve.
Imho, if the C7 will only be used as an access point only, I would suggest using OEM firmware for best speeds and compatibility.