I have been struggling with slow wifi issues on the RPi 4. At first I thought the problem might be due to incorrect settings but after spending much time I discovered that the issue only occurs when I use both HDMI outputs.
I my case I have a small 7" LCD connected to HDMI-0 and a 24" monitor connected to HDMI-1.
Wifi works OK (25+ MB/S) when only the small monitor is connected. When I have the second monitor connected the wifi download speed drops to 0.1 MB/S or less and wifi becomes virtually useless.
I experimented with different HDMI cables and cable lengths but the results were the same.
I also played with the CONFIG_HDMI_BOOST settings in /boot/config.txt by changing the values from 1 to 11 but they didn't seem to have any effect at all. Is this actually working?
Then I tried an external USB wifi adapter which worked a bit better but was also adversely effected when the 2nd HDMI was connected.
I have read that some people have experienced slow wifi with HDMI on the pi3 but there didn't seem to be any resolution.
My guess is that EMI from the HDMI port / cable is severely interfering with the wifi.
Has anyone else experienced this?