Speed will drop if you turn the router off for extended periods.
The DLM system will look at the line and in addition at looking at connection drops each houre, will also look at no sync (router off) in excess of one hour. The system will then lower the line rate and make adjustments to try to restore the connection.
If the speed has dropped I would suggest leaving the router powered on over the next 48hrs to see if speed is restored.
Please log in to My Account if you need to view or pay your bill, manage boosts and track your usage. From My Account you can also check your connection and test your line for any issues in the Service Centre.