A small script which loggs the results of speedtest-cli in a log file
Put speedlog.sh somewhere and make sure it is executable (chmod +x speedlog.sh). If you run it now, it will create a file in working directory and put its results into it.
I used systemd timers to run this script every 15 minutes. Copy speedtest.timer and speedtest.service to /etc/systemd/system/. Fix the path to speedlog.sh in speedtest.service. Then run:
systemctl enable speedtest.service
systemctl enable --now speedtest.timer
Check the status:
$ systemctl status speedtest.timer
● speedtest.timer - Test the internet connection speed
Loaded: loaded (/etc/systemd/system/speedtest.timer; enabled; vendor preset: disabled)
Active: active (waiting) since Wed 2018-05-02 13:21:05 CEST; 20h ago
Trigger: Thu 2018-05-03 09:30:00 CEST; 4min 44s left