The Archer A7 is a 750 MHz single-core mips 74Kc CPU, which isn't fast at all - especially for VPN uses (and the single CPU core also needs to service LAN/ WLAN interrupts, PPPoE, etc. at the same time), your results roughly are within the expected margins (for all intents and purposes, Archer C7 and A7 are identical in terms of performance).
While wireguard (or IPsec/ strongswan) will provide better speeds than OpenVPN, your router is way to weak to achieve anything close to 100 MBit/s over VPN.