I found an interesting thing. used Samsung S20 to test. Found that the WiFi speed is normal, without any abnormalities. But when I use my iPhone X to test, the speed is only less than 350Mbps.

The link below is the result of my test. (My ISP provides me with 500Mbps/250Mbps)

Samsung S20:Speedtest by Ookla - The Global Broadband Speed Test
iPhone X:Speedtest by Ookla - The Global Broadband Speed Test