Archer A7 has abnormally slow speeds on both Ethernet and Wi-Fi

Hello!
I'm very new to OpenWRT, just installed it on my TP-Link Archer A7 yesterday.

I installed 3 services, configured WAN to be PPPoE since I'm using this device as a router.

Here's my interface configuration (I didn't change anything except WAN, the greyed-out WAN_6 Virtual Dynamic Interface appeared on its own after some restarts)

I'm unsure why I'm getting slow speeds, now that I've installed OpenWRT:
On Ethernet, I used to get 500mbit/s - 700mbit/s, now I get 150mbit/s - 300mbit/s.
On Wi-Fi, I used to get 250mbit/s - 350mbit/s, now I get 70mbit/s - 100mbit/s (on 5Ghz)

Any help's appreciated!

Single mips core clocked at 750 MHz (QCA9563), that's about the throughput you can get from such a device using a rather dated SOC. Enabling software flow-offloading should help a bit, but the ath79 target is only good for ~150 up to maybe ~200 MBit/s - faster WAN connections require higher end devices (and north of ~500 MBit/s the situation gets rather tight).

The OEM firmware can make use of a combination of QCA fast-path (functionally similar to software flow-offloading, but implemented differently) and hardware NAT acceleration for the QCA8337N switch. While these offloading tricks can significantly improve benchmarks, it's still a bit of cheating, as it only works for parts of the traffic (the parts that can be offloaded, more complex traffic patterns (prime example, QoS/ SQM) don't profit from that at all and require raw -unaccelerated- performance).

For comparison purposes I'd always look at the unaccelerated figures, as these provide the dependable lower margin - and treat software flow-offloading as free bonus on top, which might help, but shouldn't be taken for granted (as you lose it quickly, once you're doing more complex tasks with your router).

Thanks a lot, that's very enlightening. How can I enable software flow-offloading?

I have another question: If I were to switch to another router that is more powerful, what would be a good option? What should I be looking for?