Smit, they call it 'traffic management'
During peak times, the top 5% on the Size: XL package download at least 3GB of traffic each.
Any users hitting this amount during peak times (4pm till midnight) will have their broadband speed temporarily traffic managed – their download speed will be set to 5Mb, with their upload speed set to 256Kb. This will last for 4 hours from when the traffic management policy is applied.
Even if a Broadband Size: XL user has their speed temporarily traffic managed, they can still download over 4,000 music files per day.
Most ISP's have a fair use policy, which incorporates similar measures if you read the small print.
There is also an ongoing speed issue for the XL service (although it's affecting me too on L) Info on the service status area of the customer zone.