TT XXL displays an incorrect car speed

What is more strange, it seems that the XXL "knows" the correct speed, but is slow in displaying its variations. In fact, if the speed limit in that point is (say) 130 km/h, and I go over 135 km/h (as shown by the car speedometer, also considering the speedometer systematic error), the red alarm light immediately switches on; but, at the same moment, the speed shown by XXL is, for instance, 120 km/h!! then, slowly (some minutes), the displayed speed raises to a credible value.
Anybody can help me?
Comments
Superusers
Have you tried a Drum roll restart on the device ????
https://uk.support.tomtom.com/app/answers/detail/a_id/6965//session/L2F2LzEvdGltZS8xNDI2NjM0NDUyL3NpZC9EQTdTVHpobQ
Charge the device for at least 2Hrs "OR" have the device plugged into a power supply
Press and hold the on/off button until you hear a drum-roll sound then release the on/off button.
You won’t lose anything; it’s just like rebooting your PC
I use a Soft/drum-roll start after any upgrade large or small.
It’s also useful to keep in mind for sorting out most minor niggles......
If possible, for the for the first Satellite fix, go outside with a clear view of the Sky
ATB YFM