I just cannot figure out why my Pi has such lower wifi download speed - around 65 - 75 Mbps, measured with speedtest-cli. It is connected to a 5GHz SSID. It is a new Pi model 4B 8GB RAM with an original power supply. It has Ubuntu Server 20.04 installed just a day ago and it does not run any applications. I get from my ISP 700 Mbps when connected with Ethernet cable, and between 400 - 600 Mbps from all other wireless network devices but not from the Pi. I searched for a solution and to identify the issue but I can't find it. What should I do? Thank you very much for your help and advise.
"I am expecting a speed in the same range as the other wireless devices on the same 802.11ac network - around 400-600Mbps"
This is impossible! 802.11ac might be able to reach these speeds on a good day with the wind behind it but only on suitable hardware - that means multiple antenna MIMO.
The tiny single antenna on the Pi4 will never get close; the best you could expect is 150Mbps on a 5GHz network. MagPi benchmarks show 114 Mbps.
speedtest-cli is not a reliable method of testing WiFi - it is an end to end test of internet.