When I do iperf3 measurements, the throughput in the normal and reverse direction has a difference of 50 Mbit/s. (TP-Link WDR3600 and TP-Link Archer C7 v2).
It seems like the client interface somehow causes a performance degradation?
I have noticed that. Iperf3 takes a lot of CPU itself so running it on a single-core router is going to affect the results. I think in client (and not reverse) mode it is generating the random data to send, which is additional work for the CPU.
Properly benchmarking a router and network requires fast separate machines to run Iperf3.
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Actually I use iperf3 on my laptop (so 2 Laptops). 
So
Laptop <-> WDR3600 <-> Acher C7 v2 <-> Laptop
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With an iPad and an Archer C7 the difference is even bigger. Setup:
IPERF3 Wired Desktop <-> Archer C7 <-> iPad (802.11ac@80MHz)
With 21.02/snapshot builds and standard ath10k driver (non-ct), I can get ~400Mbps downstream (Desktop->iPad) and a whooping ~550Mbps upstream (iPad->Desktop).
With a laptop the difference is not that huge: ~400Mbps (Desktop->Laptop) and ~450Mbps upstream (Laptop->Desktop).
Really weird...
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No. Ethernet connection.