Since that time our internet connection has, well, sucked.
Today the company's technicians arrived to try to find the problem. After checking this and that and the other thing they could not find a reason for the slower connection.
Downgrade back to the second-best level.
Apparently the fastest connection is known to be problematic.
By the way, upgrading is free (except for the increased monthly rate).
Downgrading costs 100 chf...