Wireguard at 200 Mbps does seem too high for MT7621. Wireguard on an ER-X (MT7621) at 160 Mbps mean is reported here.
Not sure what you mean by qca9563 being faster than the MT7621 though. The qca9563 is single core 775 MHz MIPS. The MT7621 is dual core 880 MHz MIPS. Wireguard also can make use of multiple cores. I'd be quite surprised if a qca9563 was faster.
FWIW, I found my ER-X to be speedier than an Archer C7 for SQM - don't recall specifics, but the ER-X was definitely an upgrade.