I run openwrt behind a cable modem. My provider provides me with a 500Mbit download / 25Mbit upload connection. With openwrt the speed is only 120-230Mbit.
After a restart of the openwrt router the speed of the connection increases for about 10 seconds to 500Mbit only to break in again.
I have the same behaviour when I reset all settings to factory defaults and update all packages.
I'm using OpenWrt 19.07.2 and wonder what other possibilities of diagnosis you would suggest or if you have any solutions for me.