Is smart phone's GPS accurate enough to calculate airplane speed?
That landing speed sounds rather high.  There's a regulatory maximum landing speed for airliner type aircraft, and as I recall, it's typically not far from 150 kt (240 or so km/hr).  Since it's unlikely the airplane you were in was actually flying at more than double normal speed at landing (and 180 kph will seem pretty fast to your own senses) I'm inclined to believe there was an error in your measurement.

The GPS in a tablet, reading from inside the airplane, is subject to a number of error factors.  First, even in good reading conditions, the tablet's position is generally only good to (in my experience) around 50 m; second, from inside a metal fuselage, there may be path-length errors, satellite changes, and other errors on top of the accuracy limitation of your device.

This level of positional error can add up to a large percentage error in calculated speed.

The GPS installed in aircraft have better software and can fix location more accurately, as well as more regular position checks which give more accurate speed calculation.  Those instruments make a good check against the pitot/static airspeed system.  A smartphone or tablet GPS, probably not so much.