Ubuntu has notoriously buggy Wireless / WiFi drivers. The most typical cause of slow wireless is Ubuntu’s 802.11n driver. Disabling this drive resolves the problem.
Temporary fix (not preserved between reboots):
sudo modprobe -r iwlwifi
sudo modprobe iwlwifi 11n_disable=1
Now, run speed test. If issue is resolved, do the “Permanent Fix.”
Permanent fix:
echo "options iwlwifi 11n_disable=1" | sudo tee /etc/modprobe.d/iwlwifi.conf