This is swiped from reddit but I thought it was really helpful so please don't judge me too harshly lol.
So it turns out that some Linux distros don't enable this by default for whatever reason but if you have an Intel wifi card that uses the iwlwifi driver (you can check this with lspci -k and look for a section that says Network controller: Intel Corporation and Kernel driver in use: iwlwifi under it), you can add a simple line to a config file that might make a huge difference to your wifi speeds.
Just edit /etc/modprobe.d/iwlwifi.conf (if it doesn't exist just create it) and add the line: options iwlwifi 11n_disable=8 then reboot. I ran Speedtest before and after trying this on my laptop and it seems to have increased it by about 20% or so.
Your mileage may vary of course, but hopefully this helps someone!
I tried it out on another laptop since I posted this, and that had the inverse - download speeds went up by about 20% or so, but the upload speed seems have taken a hit of about 10%. On my 'main' laptop both improved quite drastically.
So yeah definitely a 'your mileage may vary' type of situation, but it's easy enough to reverse I guess so worth a shot if anyone has a bit of a speed problem. You might get lucky!