Why do some airplanes fly at a very low speed (like this one) at a speed of 433 km/h (234 knots) at a whopping high altitude of 26,500 ft?
433km/h seems really low, but keep in mind that flightradar is far from perfect. It is not uncommon that altitude, speed or position are far off, and sometimes, you see sudden jumps to more correct values when flight radar gets new data.

You can also check other sites like flightaware for particular fligt numbers, where you get a graph of altitude and speed, and can easily play back a given flight.

You didn't post a flight date, but the flights of the last few days have a speed of 600-700km/h at that position. This is below cruise speed, but the aircraft already start to reduce altitude and speed when leaving France.