I am using a comfast e375ac with Openwrt 22.02.3. When I use the device as an Access Point and the WAN interface as a dhcp client I have speeds of up to 500Mbps. When I use the equipment as a router and use PPPoE authentication on the WAN interface, I only have 100Mbps speed despite having 500Mbps capacity, this only happens with wireless speed, connected to lan eth interface I have full capacity. Experimenting with Software and Hardware offloading there was no significant change in speed. Wireless speed drops significantly when using PPPoE.
I suspect this is related to W.E.D. (or similar for non-Mediatek chipsets).
There is some commonality between 802.3 (standard ethernet) and 802.11 frames. Which allows for relative ease in designing offload mechanisms, where essentially the CPU doesn't need to get involved for basic routing between these two media.
However once you start to do more complex routing, or add additional complexity like PPPoE, then I expect it is not allowing the offload, and all packets have to go through the processor. Not all processors are able to process data at wire-speed.. and hence would become a bottleneck.
You could look into whether this is indeed the case, and potentially try to improve the driver(s) to allow for PPPoE to be included in offloadable traffic...
Is the PPPoE/WLAN config supported by stock firmware? If so, you might want to delta the performance there. If stock firmware provides faster speeds, then you could investigate what the differences are..