When I'm descending and he reduces the speed from 300 to 250 knots, the rate of descent slows down considerably. If I'm not careful, I'll be 15 miles from the runway and still too high. I always have to use the airbrakes. Always. Sometimes the flight plan also specifies an excessively high speed of 250 knots about 15 or 10 miles before the airport. Is this a system error or typical for the A340?

Descend management is your job as a pilot. You can't leave it to the automation alone. A330 and A340 are very slippery aircraft. You can't descend and slow down at the same time. So extend the speedbrakes, that's what they are there for.