I have a 500MB + connection.
Archer C7 acts as a bridge, my NAT is done via Mikrotik.
I get 500MB+ connection when connected to cable.
My laptop (having AX adapter, and my phone Galaxy S21 Ultra) can't get speeds faster than 300MB.
They are in close proximity - about a meter.
Mode: AC, Channel: 36, Width: 80
Maximum tx power: driver default
Encryption: WPA2-PSK/WPA3-SAE Mixed
802.11r setup with an older TP-LINK.
Phone shows 866MB connection, CPU load when doing speed test is about 1-2%.
Your phone probably only has two wlan antennas, meaning 802.11ac/ 2x2 --> 866 MBit/s (theoretical link rate) is the maximum (with any 802.11ac router), with 300-350 MBit/s ending up on your device in practice (the archer c7 is a bit slower than contemporary 802.11ac/ wave2 device, so ~300 MBit/s sounds sensible). For faster connections, your client either needs to support more wifi streams (3x3/ 4x4), which you won't get with common phones/ notebooks - or you'd need to move to 802.11ax routers (which can scratch- and cross the 1 GBit/s barrier in practice).
I don't know which AP I was connected to, but was able to get speeds up to 600MBs on this phone in a hotel.
so it's not your phone, but your C7.
For sure it's not the phone
I think here is a lack of information. To know if you are measuring something reasonable, my suggestion is measure at least WiFi RSSI in both sides, SNR, MCS modulation and channel ocupation in each scenario. And of course be sure that the WiFi is the bottleneck (no the speedtest server, for example).
I think that there is no way to ensure something with "one day somewhere this was 600 Mbps and now here is 300 Mbps"
Honestly, in my experience, 300 Mbps I think is a nice value for a 2x2 80 MHz 802.11ac in a "real case" scenario (as you are not in an anechoic chamber). But this of course still being just an unproofed affirmation because I am talking about "my experience" and not concrete and complete measurements.
In LUCI in "Wireless" you have an easy way to obtain almost all the information I suggested to look:
I gave up. The C7 was causing problems with latest firmware.
Just ordered a AX router, flashed OpenWRT and now get speeds of 500+ MBps with the same laptop and phone.
The bottleneck was the C7 for sure
Mi Router (Xiaomi) AX3200. Mine had telnet unlocked, so that I was able to flash OpenWRT.
I'll sell the C7