I assume it keeps going as long as Ofcom are willing to keep paying them. I'm not aware of any end date and have recently had my box replaced as the old one kept crashing - so if they are still sending out new boxes I can't believe it is ending in the near future.
Mine hasn't impacted on speeds. You could have an issue with the box though as if you have set it up to their instructions all of your network traffic (barring wireless) will be traversing the samknows router and so any problems can stop things (or slow them down).
The old box I did occasionally get sudden speed drops solved by a reboot of the samknows box.