Your downstream noise margin is 10dB, which is high and suggests the target noise margin has been increased due to instability on your line (the standard target is 6dB but most stable lines will reduce to 3dB). This is almost certainly the cause of the reduction in sync speed.
Have you noticed the connection dropping regularly? The second screenshot shows the connection dropped 1h 32m before the screenshot was taken. Connection dropping regularly suggests a line issue, either inside or outside the property.
Alternatively has the router been frequently rebooted or phone line disconnected? Try to avoid this as the system interprets it as instability.
If the underlying issue is fixed then the target noise margin (and therefore sync speed) should correct itself.