For what it is worth. Advice by BT customer help is as follows:-
Every time you switch off or re-connect your router or modem, your broadband service refreshes your connection speed. Note, however, that where this has increased, your throughput speed will not increase for a further 3 days. This is a to minimise the risk of data loss which would occur if the exchange tried to send data down your phone line at a faster rate than it is capable of handling.
To avoid this happening, your exchange waits for three days of consistently higher connection speeds before it will increase the speed at which it passes data down your line. Also note that if your connection speed dips for any reason during the three-day period (eg. due to electrical interference or a fault), the exchange will re-start the three-day period.
For these reasons we recommend you leave your router switched on all the time, and avoid measuring your throughput speed until three days after you last switched on or re-booted your router or modem.