Archer C7 and LAN performance

You could run iperf3 on the router as server and test with the PC as client to check your LAN connectiviy.
Also check the CPU load with htop on the router.

# opkg update && opkg install iperf3 htop

Start the server.
# iperf3 -s -V -D

Start htop to monitor CPU load when you speedtest your Internet or your LAN:
# htop

Start htop as client on the PC
iperf3.exe -c [IP address of your router]

These are the results from my PC with a Linksys WRT1200AC.

iperf3.exe -c 192.168.90.254
Connecting to host 192.168.90.254, port 5201
[  4] local 192.168.90.220 port 53523 connected to 192.168.90.254 port 5201
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-1.00   sec   112 MBytes   936 Mbits/sec
[  4]   1.00-2.00   sec   111 MBytes   933 Mbits/sec
[  4]   2.00-3.00   sec   112 MBytes   933 Mbits/sec
[  4]   3.00-4.00   sec   111 MBytes   936 Mbits/sec
[  4]   4.00-5.00   sec   111 MBytes   935 Mbits/sec
[  4]   5.00-6.00   sec   111 MBytes   934 Mbits/sec
[  4]   6.00-7.00   sec   111 MBytes   929 Mbits/sec
[  4]   7.00-8.00   sec   111 MBytes   935 Mbits/sec
[  4]   8.00-9.00   sec   111 MBytes   935 Mbits/sec
[  4]   9.00-10.00  sec   111 MBytes   929 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth
[  4]   0.00-10.00  sec  1.09 GBytes   933 Mbits/sec                  sender
[  4]   0.00-10.00  sec  1.09 GBytes   933 Mbits/sec                  receiver