Archer C6 v2 openwrt AP mode switch speed

At the moment with factory firmware it can handle 1gb up/down in AP mode/switch.
My question is will it also handle 1gb (in AP/switch mode) if I flash openwrt and configure it with a few vlans (2-3)???
Will that extra tag stripping effect the speed?

Stock firmware uses hardware NAT.

OpenWRT doesn't, so it will affect speed.

If you are using the device as a dumb ap (and not routing) you should get full speed on the switch.

NAT (and hardware accelerated NAT) applies only to routed (L3) connections. Switched (L2) connections, including forwarding VLANs will run at full speed on the dedicated hardware switch chip.