i had this issue when i first setup router i used qos bandwidth limiter for certain ports but this cut my bandwidth using speed tests in half so i reset router and didnt use qos i havnt had a problem since.
ive just purchased this router upgrading from bthub which gave full speed 900mbs the router started off ok at full speed but a day later was down to half speed tried merlin firmware but the same keeps happening until i do factory reset.
